I don’t think there should be any rules about the length of a date, except that if it feels okay, keep going. But if it feels okay and yet she’s saying thins like, “It’s getting late” or “My parents wanted me home by midnight” or just shifting around in her seat gazing around the room, then you should stop and take her home. Because she’s probably just being polite and making it feel okay when really she is mentally putting a pistol to her temple.
Of course, some people think there should be rules about how long a date should be. There always are people with rules. It’s only a problem when the people with rules are the people you’re dating.