The phrasing heavily implies that they worked up their nerve over the course of three hours and that this was a recurring conversation throughout that time. And I'm not assuming they were attentatively listening; what's problematic is that they leveraged (unsuccessfully) what they overheard to swoop in like a creepy vulture and try to pick this woman up (under the guise of doing her a favor) while she was just trying to have a nice time with her friends.