To wander through and get a good look at the lights - you need about 15 minutes. (And make sure you wait long enough to see them "dance.")
To stop and take photos, maybe have your picture taken by a Photopass photographer, really just stop and enjoy the atmosphere - maybe an hour. I think the crowd varies by the day of the week, the weather, and the time - but I've been there when I think it's "pretty crowded" - but as long as you aren't waiting in line for the perfect photo spot with a Photopass photographer - I don't think the crowd level influences your experience much. As long as you are ready for the slow meander walk and realize that some folks will be STOPPED and you need to walk around them, I don't think the crowd influences your time much. It's really about how much time to you need to take them in.
The crowd makes it festive (community celebration), but when it's almost empty - the lights feel a bit more magical - as if they are special for the select few. I've seen them both ways and liked both for different reasons. If you want the lightest crowd - you have to stay until the park is closing out and a bit after.