Hey there!
So I would say yes 30 minutes is gonna be a pretty hard maximum. I think if you were to go over a little bit then the absolute most I'll allow is an extra ~2-3 minutes if necessary, but no more than that.
I want to make sure that as many people as possible will be able/willing to listen to plenty of submissions and give feedback, and I feel that allowing people to go longer than ~30 minutes will hinder that.