Depended on the army, of course, but there are two historical misconceptions behind your question:

1. That, unless killed or injured, a soldier would enter a trench in August 1914 and not leave it until November 1918. Most units were rotated through the trenches, regularly given rest time and so on. The British Army, for instance, generally had three Batallions per Regiment, of which one would be in the front-line trenches, one in the support trenches and one behind the lines resting/in reserve. They would switch around anything from every few days to every few weeks, depending on how intense the front line was. On average, a British soldier would spend about 10 days per month in the front line trench.

Beyond that, units which had seen combat would often be sent back behind the lines to rest and refit, absorb replacement men and so on. Units were often switched into quieter sectors as well, while "fresh" units were cycled into the trouble spots.

2. That the casualty rates in the trenches approached 100%. Again from British figures, 700,000 men were killed in action, out of 6 million mobilised - about 11%. I'm not sure whether this figure includes "died later of wounds received" but nonetheless it's lower than most people assume.