First of all 2 and a half hours is WAY too long. Ouch! From everyone I have talked to in dental offices, friends and my own personal experience 30 minutes is pretty much all your should go at a time with those. Everything that is going to happen with your teeth being bleached pretty much happens within that 30 minute session. Bleach is like any other chemical. It will go into your bloodstream through any tissue. Your skin and inside tissue is the largest organ you have. I have to agree with the other poster, it could effect your nervous system. That is kind of scary. And most definately it could make your teeth extremely sensative. It can even take of a layer of enamel. This happened to a friend of mine. She left her tray in over night. Not good!
I am sure she will be fine but I wouldn't let it happen again. Good luck!