https://www.reddit.com/r/AskHistorians/comments/4glr0c/what_were_your_odds_of_surviving_going_over_the/

u/duxbelisarius has a good answer to this one

I would add that on average a soldier in the British Army spent one month in the front line trenches, and two in the support trenches. There really isn't a measurable means (that im aware of) to know how often any individual was likely to go over, only because it was dependent on many factors. On campaigns, more likely, but you're rotated to a support trench? Unless the enemy attacks you it's not likely at all. Some vets could over a few times from a major campaign or raids, and others could spend months in the trenches not involved in a full scale assault.

We are taught to think that soldiers going over the top in the trenches would have been mowed down straight away.

I'll I add to my comment in the thread linked by u/eastw00d86 to further illustrate how this is a falsehood. The example I gave of ~50% casualties suffered by the BEF on the first day of the Somme needs to be balanced by the fact that this was the worst day in the history of British arms.

As a counter example, at Vimy Ridge in 3 days during April of 1917, 4 Canadian and 1 British Division incurred somewhat more than 10,000 casualties from a total of 170,000 men engaged. Fatalities over 2 days were therefore a little over 2% with a further 4% wounded or otherwise injured.

Again, while the Newfoundland Regiment suffered 658 casualties out of 780 men on the first day of the Somme, a Brigade of British and Indian cavalry engaged with considerable success on the 14th July incurring only eight fatalities and around 100 wounded out of a total of 1,500 men - a fatality rate of 0.5%....

The overall per diem loss rates were therefore not necessarily excessive in the case of a major battle, with the overall fatality rate for the BEF in France and Belgium at somewhere near the 10% mark over four and half years of war.

We also need to understand that major battles were not the usual state of affairs for the forces engaged.

The British operated a rotation policy with a Division in the line having one Brigade in reserve and two Brigades "up" at any given time.

Of these two brigades, two of the four battalions would be "up" with one in the second line assigned to work parties and the fourth battalion at rest.

Drilling down even further, each battalion would only have some of its companies up with the remainder in the second line.

Gordon Corrigan calculated that the actual time at the front for a battalion not engaged in battle was less than nine days a month, it being unusual for a Battalion to be in the trenches for more than five continuous days, and in the actual firing line for more than 2 consecutive days.

In addition to this program of rotation, entire Divisions could be removed from the line en bloc for rest, retraining, or reconstitution.

The myth that the Great War consisted of nothing but suicidal charges appears to be largely due to the vast numbers of casualties incurred.

it’s true that average daily loss rates are staggering to comprehend.

During the Somme, BEF average daily loss rates were 2,950. At Arras in 1917, the number was 4,070, during 3rd Ypres it was 2,121, and during the Hundred Days offensive it was 3,645.

Yet these figures are meaningless unless we put them into the context of the total number of men deployed. No one would say that a daily casualty return of two to four men would be excessive for a force of a thousand men engaged in a major battle.

Yet in the Great War, we see thousands of thousands of men engaged, and thus the casualty rates are unsurprising. Indeed, as John Terraine has argued, to expect anything less is a delusion.

Sources Kenyon D, BRITISH CAVALRY ON THE WESTERN FRONT 1916-1918

Gorrigan G, "Mud Blood and Poppycock"

Terraine J, "The Smoke and the Fire"