Ask the drive-in theater for their recommendation.

If you're going to the Goochland Drive-In Theater (which has a really buggy website) this is what they say:

>Plan to arrive early.....arriving 1 hour before showtime isn't considered early. All attendees under 1 ticket must arrive in the same car - no later drop-offs/walk-ins. Gates close 20 minutes before showtime....never wait until then to arrive. Once gates close - no later entry is permitted - sorry no exceptions

Basically, you should arrive between 20-60 minutes from showtime, but absolutely no less than 20 minutes before showtime otherwise you're not getting in.

For Goochland specifically, they only do advance ticketing and do not sell tickets onsite, so there's no risk of you getting there 20-60 minutes beforehand and finding out there's no space for you despite you already having a ticket for your car.