This is the best way to get plenty of insight into what a particular girl thinks about certain activities. If she loves it, you can go back another time, but not the next time. Perhaps the most difficult part of any date in GTAIV is the end, when you're bringing a girl back to her place (likely swerving all over the road and killing innocent bystanders since, y'know, you're drunk).
The game will give you an option to say goodnight or to try and go into the girl's place with her.
Usually, from the point of cellular contact, you're going to have an in-game hour to get to your date before she gets miffed for standing her up.
The date can still go on to a certain point even if you're late, but it's definitely not the way you want to get things started.
If it went well, then you can expect another date, but even girls subjected to really bad dates can be salvaged. Wait a few in-game days at the very least before you call a girl for another date, and be sure to keep an eye on your relationship meters to see how much a girl does or doesn't like you.