Firstly, it needs to be understood that "battles" like the Somme were actually offensives comprised of many separate and individual battles.

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

As to how much time you would spend in the front line, It depends which army you were in.

I can only really speak for the British Expeditionary Force who operated a rotation system at a number of different levels.

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.

Troops in the second line would be assigned to duties like bringing up rations and ammunition, repairing or building fortifications, burying cables and so on. This was hard work and as it was mostly within artillery range, was hardly free from risk.

Troops at rest would typically be afforded hot baths, have their uniforms laundered, and set free on the locality to spend their King’s shillings on food (egg and chips being a favourite), beer, and women. The tins of plum and apple jam that was so ubiquitous a feature of their rations being used to seduce fair young maidens.

The rotation system was seriously disrupted in the winter of 1917/1918 when Lloyd-George withheld reinforcements from Haig, meaning that Brigades had to drop from 4 to 3 battalions. The effect on the rotation system was disastrous at a time when all hands were needed to switch from the offensive to the defensive following the collapse of Russia and the impending Germans spring offensive. In the words of one officer, his men had become a collection of “deskilled ditch diggers and cable buriers”

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

As Paddy Griffith points out in Battle Tactics of Western Front Divisions perceived to be reliable were more likely to see more action than those perceived to be unreliable.

Yet Charles Carrington noted that in 1916 he spent 65 days in the front line, 36 in close support lines, and 120 days in reserve positions within a day’s march of the front. He spent 73 at rest, 10 days in hospital, and 17 days on leave.

Between December 1914 and August 1916, James Jack recorded that he spent 141 days in the trenches, 130 in reserve, 15 days travelling, 19 in hospital and 12 on leave.

As we can see, men did not tend to spend months on end in the trenches.

The notable exception being Brigadiers. As the most junior generals, they received no rest while their brigade was up, even as its constituent battalions were rotated out. The psychological strain of command and constant danger (Brigadiers were more likely to be fall victim to an enemy bullet than any of their subordinates) took a massive toll on these usually middle aged men.

The rotation system was not cheap. It was a colossal headache and a logistical nightmare for the staff officers who had to plan and implement the constant cycling and movement of men up and down the front, who occupied scarce road space and transport which was sorely needed for ammunition, food, and water.

But the rewards were obvious. The rotation system ensured that morale of the BEF never broke in the same way it did for the French, Russians, and Italians in 1917, or the Germans in 1918.