Not tours so much as rotations. They would rotate through the trenches and then be back behind the lines where the could train gets some R&R and if lucky gets some leave too. Going home on leave was easier for the French and Germans but British soldiers could get home too. Check out this great BBC piece about life in the Trenches. The best short read I've ever found on the topic.

As for finishing your tour and going home. That was definitely not an option. For a British soldier the shortest time you could join for was 3 year or the end of the war whichever came last. British enlistment during WWI