Conditions varied greatly depending on where you were, and what the conditions were like in which sector. In Flanders, where the water table was quite high, 'trenches' actually tended to be earthworks or redoubts of some sort built upwards from the ground, while in the south in the Argonne and the Vosges there weren't trenches at all, but individual redoubts cut out of hillsides. Both sides (Franco-British and German) had rotation systems, with German divisions in general spending more time on the frontline. British battalions could expect to spend an average of 4-7 days in the actual firing line, and the rest of their time in the rear or in reserve. Though it is true that German trenches tended to be better, this had to do with differing philosophies and roles, [which I discuss more in detail here.] (https://www.reddit.com/r/AskHistorians/comments/3cviro/i_often_hear_that_during_ww1_the_german_trenches/) In the winter they were cold, when it rained they could be muddy and wet, but in general the soldiers and armies learned to cope and adapt.

On top of the difficulty of making generalizations, when the trenches were so diverse in condition, is the fact that the 'trenches' really only existed as popular memory has it from 1915-16. The experience of the fighting in 1916, especially that of the Somme, showed the danger of holding static defences for the Germans, in the face of Franco-British air and fire power. The result was that 1917 saw the use of offensive tactics that rendered the trenches obsolete, while the defender moved to a system of elastic defense in depth that rendered trenches irrelevant in combat, essentially.