Shelf Life of Gels...

I feel like I'm afraid to know the answer here haha. My collection has grown quite a lot since I started this hobby! Most bottles and pots are opened/swatched. How many years do you find you get out of a single bottle, opened and unopened? I know it's technically 12-18 months but that would mean literally half my stuff is now technically out of date, and that makes me want to cry $$$ !!!