I think it's personal for everyone....I would stop at 35, just because that is my cutoff....plus I have a strong family history of VERY early menopause (in mid to late thirties) and cervical cancer history...so I guess I figure I'll either go through menopause and not be able to get pregnant and I have a fear of getting pregnant and having a cancer diagnosis at the same time.....perhaps I'm just paranoid though...
I have many friends that have conceived in their late thirties or early forties and I think it is a personal choice for everyone, I'd say keep trying until YOU feel like it's time to quit, don't let anyone dictate that choice!