I saw a news special that said that 2 week contacts were the exact same construction as one a days. The contact manufacturer, Baush & Lomb with Accuvue lenses. This was over 20 years ago. I then proceeded to wear my one a days like 2-3 week sleep in contacts. After going to various checkups through those years, I was told my eyes were fine. Until the last exam, where I was advised to change to BioTrue lenses. And they were so much better feeling.
My prescription actually went down that exam. From 3.5 in the left eye to 3.25 in the left eye and from 3.25 in the right eye to 3.00 in the right eye.
Within a year I had massive pain in my left eye that was attributed to sleeping with contacts on. Solution was some medicated drops for a week. I didn't wear contacts for a month after that but have since gone back to the same ways. No problems.
TLDR
: Moral of the story is you can wear contacts for way longer than prescribed, at least in my experiences. And unless it gets bad you can continue to do so.