Generally speaking, pretty much everyone should bail on a minimum of 4. Note that bail truly means bail from the beginning—you don't actually try this problem in the first place. You'll have other questions that you try, they don't work out, and then you have to guess in order to move on. Those ones don't count in the discussion we're having right now. (And for the try-doesn't work-bail scenario, don't count. There's nothing you can do anyway—you already tried!—so just get out and start working on something else.)
If you're going for a score of Q49+ or V 40+, I'd bail on 4 to 5.
If you're going for a score of Q40 to 48 or V 30 to 39, I'd bail on 5 to 6.
Lower than that, 6 to 7.
Also (for anyone else reading), if you find that your timing issues are mostly in one section, you can adjust the above accordingly. But pretty much everybody is going to see at least a couple of "aagh this one is terrible!" problems. Don't raise your blood pressure or use up valuable brain energy even if you do have the time. Business decision—bad ROI. Get out!
By the way, on my last real test (in January), I bailed on 8 quant questions. I went into it wanting to perform the following test: any problem that made me think "ugh this sucks" as I was reading it, I bailed. Immediately—didn't even try to make an educated guess. I picked answer B all 8. I still scored a 48. So if you find yourself having used up all your bails but one more awful one comes along, go for it. By which I mean: bail again!