For concerts, how long after doors have opened do they stop letting people in?

you can go in until the concert ends

They seem to be very flexible about entry

Most of the time on concerts the doors remain open untill the last 20-30 minutes of the concert itself, unless otherwise requested by the artist.

In my experience at this venue they never stop letting people in.

Really up to the event. Rock concerts 30 minutes .but why hang out in the lot. I've done 9 (sports arena) BLACK SABBATH at 14

If it's a fire hazard they will stop letting people in.

Get there early and not worry about it