EDIT: I am home now and have provided a more in depth answer as a reply. But here is the TL;DR: The quick answer to your question is that it depends on the injury you sustained. Surprisingly if you were wounded and received treatment you had a 95% survival rate(Found under "American medicine in the second decade..."). If you were hit by artillery you had a 30% survival rate. If you had a limb amputated you had a 95% survival rate. Survival rate of diseases with treatment was 30%. If you were wounded on the battlefield and were beyond help (head wound, abdomen wound, artery hit), you were left there to die on the field.

I study 20th century warfare so I’ll try to give the best answer that I can. Right now I am on my phone so I can’t pull up exact numbers but once I am home in an hour or so I’ll expand on my answer and go into more detail. For now I will answer your question specifically to the question you asked. And most of my answer will include more influence from the US history. But once I am at my desktop and have my books I will be able to provide numbers and percentages on survival chance dependent upon injuries.

To get to your question, the answer to your question depends on a multitude of circumstances; so it’s difficult to answer. A number of questions have to be asked to determine your chances of survival.

Where in no man’s land are you? Are you 5ft from your side’s trench or are you 200ft from it? Did you end up next to a soldier nearby to assist you? The location in which you were injured greatly affects your chances of survival. The closer you were to being able to receive aid, the time it takes before a medic begins to work on you, these things will help determine your chances of survival. Back in WW1 there was no modern day corpsman. Where there would be a trained soldier having all the tools and equipment to field stabilize someone. Durning WW1 the US in essence borrowed from the English and French system. Medical staff used stretchers to run out and grab people and bring them back to their side where they would be transported back to the med tents. And then medical staff would begin work on you. Additionally, if a med tent became overflowed with wounded it could take more time for you to receive aid, lowering chances of survival. If there was prolonged machine gun fire or artillery, it may have not been possible for you to receive help as it may have been to dangerous for them to get to you. There could be some instances where some limited immediate help could be provided. Let’s say you got injured and ended up in a crater left from artillery and there was someone else in the hole with you. They may be able to apply a tourniquet and bandage to your wound. Thus your chances of survival would improve. Or maybe you were shot immediately after exiting your side’s trench. Someone may be able to pull you back into the trench and you’d be brought back to the med tents. Like I said, right now I don’t have access to the numbers so I apologize. But if you were wounded in the trench and were brought back to the med tents for better treatment, your odds of survival would greatly improve.

What kind of damage did you sustain? Were you shot in the end of a limb? The chest? The head? How much blood are you losing and how long will it take for you to bleed out? The severity of your wound is another factor that determines your chance of survival. In WW1, the medical technology in combat could even be considered primal when compared to today’s and even WW2’s medical standards. The more severe of a wound you received, as applies to any situation even outside of war, the lower your chances of survival. But it was even worse in WW1. Since field medics weren’t standard practice, bleed out rates from bullet wounds and missing limbs were high. When I am home I will provide numbers on survival rates from different types of wounds and injuries sustained.

Another issue that soldiers faced were the low sanitation standards in the trenches and in med tents. Sometimes in trenches it was like standing in marsh water. People were sweating in the water, bleeding in it, throwing up in it. It was an absolute breeding ground for many diseases and illnesses. Let’s say you received a superficial wound like a graze from a bullet or shrapnel, or you got cut somehow. The injury you sustained may have not been enough to send you back so you continue to stay on the frontlines in the trenches. Now the wound you received is being exposed to all the bacteria that is thriving in those trenches. And then you get incredibly sick. The medical technology wasn’t their yet at the time, getting sick like that could mean certain death. This sort of thing even more so extended to the wounded in the med tents. In WW1 the medical practices had almost barely evolved since the Civil War. It wasn’t fully understood by nurses and doctors at the time the importance of sanitation and keeping things sterile. Salt water was the main sterilization technique used to clean wounds. And once infection set it little could be done which resulted in high numbers of lives lost. Illness was so bad that it claimed more lives than gunfire did.

I apologize that was my answer was not incredibly extensive and as I said, I will elaborate on it when I am home. I hope this somewhat satisfies your question.

If you want more information on this right now, read this: https://www.ncpedia.org/wwi-medicine-battlefield

The scenario leaves a lot of questions regarding assumptions. Are you asking how likely you are to survive simply being in No Man's Land, getting back to the trench, or long term (the wound, the war itself, etc)?

I'll try to address each of these in turn.

First, let's take a look at when you would have gone over the top. It's either part of a trench raid or probe, or it's in a larger attack on the lines.

If the attack occurred as a trench raid or probe, it would have happened overnight. You could have been strafed by machine gun fire and clipped, and possibly separated from your raiding party. In this case, you'd need to be able to very slowly pick your way back to the lines. Dangerous, but possible - and this did happen. There could be occasional illumination flares, and that (along with noise) could be the most dangerous time to be caught in the open. Soldiers would try to memorize the shapes and shadows of what they could observe at night, because the movement of the flares would shift the appearance of the shadows, causing the appearance of moving objects. When observing a part of the line, a soldier would be aware of the optical illusion caused by illumination flares and would try to spot anything out of place. Noise near the enemy trench might invite a harassing grenade. You could also easily run into an enemy raiding party, or even another friendly raiding party, although that's a little less likely since the communication was pretty good about who was going out for raids. Still...very dangerous. Pick your way quietly back to the line, whisper to your mates, and hope the enemy doesn't hear the commotion. On the other hand, it's also likely that your buddies in a raiding party (there could be as many as 20) would spend some time searching for you before they gave up. They'd bring you back if they found you (alive).

Now let's examine what might happen if you were clipped by a bullet or shrapnel during a larger attack on the lines.

Unlike trench raids, dawn was the standard time of attack. This means that if you went over the top and were wounded, you'd have a helluva long wait until dusk to get back to the line unnoticed. However, it's kind of a Hollywood assumption that this is what would have to happen for you to get back to the relative safety of the trench.

Maybe you can pick your way back. The gap of No Man's Land looks different depending on where you are. The average width is 250m, but the gap could be anywhere from 50 to 500m. The closest was SEVEN METERS at Zonnebeke.

Near each combatant's line, there could be as much as 100 ft of barbed wire in many bands. There would be water-filled shell holes. The terrain is pockmarked and difficult. There could be primitive land mines of sorts and unexploded ordnance. If you could walk, you would. With luck, you'd make it back without being hit again. Or caught in wire. Or stuck drowning in mud.

Let's ground this for a moment: there was quite a bit of casualty collection in No Man's Land, both formally (organized stretcher bearing parties) and informally (buddies carrying buddies during battle). If you were hit during a battle, you MIGHT be carried back right away, depending on the wound. There were also frequent, informal ceasefires arranged to allow for stretcher bearer parties (including Red Cross) to enter and remove wounded. Depending on where you get stuck in No Man's Land, you might be collected and treated by the enemy - or get swapped between friendly/enemy stretcher bearers and get back to your side for treatment.

Simply being in No Man's Land is very dangerous. If you are there at night, you risk being caught as a trench raider by the enemy, hurting yourself on any of the very dangerous things on the ground, and getting killed by your own side. During a pitched battle, you risk getting hit by machine gun fire, a stray or directed rifle bullet, or shrapnel if there is an artillery attack or someone tosses a grenade (note: use of grenades in WWI is more of a harassing feature than how they have been used in more recent conflicts, beginning with WWII).

We've talked about the possibilities of getting killed in No Man's Land, getting killed while trying to regain the relative safety of the trench, so now I'll turn to the longer term questions of survival.

Let's say you are among "average" British troops. The casualty(dead and wounded) rate is 56% and the over all death rate is 12.5%.

A significant number (not quite 30%) of simple leg or arm wounds could result in death. American statistics show that as many as 44% of any gangrenous wound would result in death. A head wound puts you at 50/50 chances, and an abdomen wound is near certain death (99%). A majority of wounds came from shells. This should not be a surprise, since there was a lot more shelling than small arms combat. Since shell shrapnel can carry debris into a wound much more so than a bullet, you would be more likely to receive infection from a shell wound. If you're dragging yourself through a muddy No Mans Land with dead bodies all over the place, you can expect that any wound you have received could get infected.

In summary, a lot depends on the nature of the wound (of course), the time of day, the type of attack, the distance to friendly lines, the location of the wound, whether or not it gets infected (including time to treatment), etc.

Lots of resources available upon request. I will also recommend for general reading on the experience of day to day life and fighting in the trenches, Herbert McBride's "A Rifleman Went to War."

As a follow up question: How would a wounded soldier get back to his trenches? Did the opposition allow for recovery of wounded? That seems unlikely. If the wound is more severe then it could be impossible to crawl back.