My Mum and Dad had about 9 years between them, and at the time she was 18 and he was whatever age. It lasted about 5 years or something, and I think amidst all the arguments and their incompatibility it was ultimately down to them wanting different things. While she wanted to go out and have fun with her friends, he was content with sitting in night after night.
If it works it works, but Im guessing you're no older than about 24, most likely younger, so the age gap could cause difficulties due to you wanting different things from the relationship.
Saying that, no-one can possibly tell you how long it will last, though if you're having doubts (as I suspect, considering the nature of your OP) then maybe re-evaluate the situation?