I think it all depends on the person and the brand of lenses, the prescription, etc...
I wore a different brand (monthly) then the ones I wear now and within a few months of wearing them all day, everyday, I had corneal ulcers.
Changed brands, change the lenses every 2 months, take them out and clean with a hydrogen peroxide based solution every single night, use a very good re-wetting drop, and let my eyes rest 1 day a week from contacts and I have no issues whatsoever. If I don't follow these instructions to a tee, I have issues again.
Jealous of those who can leave theirs in forever and not have issues!