95% effective after 24 hours.
Drops to 89% effective by 72 hours.
It's murder on chicks' bodies, too. Like a really terrible period.
And you can definitely still knock her up on it if you wait till the 72-hour mark anyway... ask me how I know! (well, don't ask, but yeah -- 72 hours is far from guaranteed)
@James D-
If she has a regular cycle, the odds of pregnancy the first few days of her period are near-zero.
But still not totally zero. You can get spontaneous egg release if you get her super, super excited (huge fight + makeup sex; you returning from travel and she's thrilled to see you and has been anticipating it; etc.).
If she has a shorter-than-regular cycle, the odds are unpredictable. I'd figure out how regular or irregular her cycle is before taking any chances, unless she's baby mama material and you're chill with it.
Chase