Depends on the girl. Some girls have longer routines that they tend to shorten or skip through some steps if you went for a much shorter session like 30 mins.
I usually go for 45 mins. I find that length of time to be perfect for 1-shot. The girls wouldn't be rushed, no detectable difference in service level compared to 1 hour session.
Just try to tell the girl at the start of the session that there's no need to rush, and that you only want to do 1-shot.