My first REO from 2013 is pristine and I vaped it almost exclusively for years. Same for my provari of the same vintage. Heck my old original protank still works (yes I still have coils for it).I expect most mods to last a lot longer than we think. I've heard people allow for them lasting 1 year. But I have mods I bought in 2015 and 2016 that I'm still using and they work as well as when I first bought them. They may look a little rougher, but they vape the same. The only 2 mods I ever had that quit were internal battery mods. I do have a lot of mods stocked up, but I'll probably never need many of them. It's nice to know that I have enough to feel comfortable. However, I'm at home most of the time, so there are less mishaps that come when you're out and about. So your mileage may vary.
That said I do have a couple of mods I don't expect to hold up too long. My 2 SMOK Aliens have been workhorses for me but I have heard others have had problems with them so I sort of figure they will quit fairly soonish.
Overall the vast majority of my collection is mechs. I really don't expect too many issues from them that I can't fix or find a friend to fix.