If any at all? Unless of course they are drunk, then they'll stare at guys like guys stare at girls.
Most Helpful Girl
I can think of three reasons off the top of my head.
1. If they like you and they're shy, well... there you go, they're shy and can't maintain long eye contact.
2. They don't want to come off as intimidating and potentially scare off the guy, if they like him in the first place.
3. They don't want to give the wrong impression to guys. Some guys might take the long eye contact as a signal that the girl likes him when it might not actually be the case.1