I can't sympathise with you as I was lucky enough to get pregnant on my first IVF, but after 3 failed IUI's and 5 years of trying on our own, I have had enough BFN's to partially understand.
To answer your question, I would say go on as long as you feel you can. No-one can put a number on how many cycles is too much. If your body and spirit and that of your partner can carry on and your desire to have children is greater than the trauma of another BFN, then chin up and strive forward. Who knows...attempt 6 might just be the one.
If your RE thinks that you should be getting pregnant, then surely its only a matter of time....that would give me hope enough!
I wish you the best of luck,
Em
TTC 5yrs - Unexplained Infertility
3xIUI's (April, June & Aug 06) - BFN,s
1st IVF - Oct 06 - Jasper & Finlay born 07/06/07
Natural preg - Addison Grace born 15/10/08