This following info comes from a book I'm currently reading "The Western Front Companion" by Mark Adkin.

It's a misconception that soldiers in the first world war spent most of their time in the front lines. The BEF (British Expeditionary Force) took great pains to ensure that men and units rotated regularly between front line positions and billets in the rear. In January 1916 for example, the average time spent in the front, support or reserve trenches was 6 days, with a maximum of 4.2 consecutive days in any one trench, and a maximum of 3 consecutive days in the firing line. Some small units like the British Gurkhas spent more time in the front like because there were less men to rotate, one Indian battalion spent three weeks in the trenches.

The French relief system was much more haphazard and unstructured until after Verdun, and was a partial cause of the 1917 mutiny.

I think SovietSteve has covered the big misconception pretty well. Men did not stay in the front line for longer than a few days apart from in very isolated exceptions. I would just add that if they took part in attacks, as opposed to simple trench-holding, the period in the front line would be even less. So most of the men who went over the top on the famous 'First Day of the Somme', 1st July 1916, were back behind British lines by the end of the 2nd. Here's a good overview of how the British system worked.

On a wider scale, it was generally in the interests of the generals to move units from one sector to another and make the most efficient use of their forces. It didn't help to leave a division/regiment in the line until it was exhausted if there were well-rested units which could replace them. The way the French rotated units in and out of Verdun has already been discussed. For the British, who never had enough troops, this was even more important, so often divisions which formed the reserve for attacking units were soon after deployed themselves for follow-up attacks. British units didn't really have any geographic ties to where they were serving and additionally British corps and armies were purely administrative, so it was very easy to shift units around to where they were needed. The French system was less flexible and left units in the same area for long periods, which was one of the factors which contributed to the 1917 mutinies. For the British there were also factors of quality to consider; with four distinct waves of troops being deployed, the regular army of 1914, the Territorials who made it to France beginning in early 1915, the New Armies who made their debut on the Somme and the conscripts of 1917-1918, it made sense to deploy the new, less experienced troops in trench-holding and the more experienced troops in offensives. This is part of the reason why, as is commonly claimed, the regular army had practically ceased to exist by the end of 1915. This applied too to American troops; while they did perform well on the attack, most units ended up holding quiet sectors to free up more experienced French and British units for major operations.

I think its also worth remembering that not every soldier spent his war in a theatre of war. Many men were stationed in India or the Caribbean (for example) on garrison duty.

I've also seen many cases of chaps being wounded in theatre and then being transferred to the regiments garrison battalion for less strenuous duty.