If the cars battery is fully charged and you leave the headlights on and go do something such as shopping and forget to turn the headlights off the battery can drain enough or die enough to where the car won't start in as little as 45 minutes to 60 minutes.
I once forgot to turn my headlights off on my truck and went into homedepot shopping and 50 minutes later I came out and my lights were dimming.
The battery was too low to start the truck back up so I had to have someone jump start my truck.
So if the headlights are left on the battery can die within 45 minutes to an hour.
If it's your interior lights that are left on in the car they tend to draw less power so the battery may not die until 2 to 3 hours later.