We don’t seem to have the whole story yet, but I believe it was under 20 minutes.
He seems to have fired at least 100 rounds, and I think it may have been more.
What the rate of fire was in any given period was, or what opportunity the victims had to escape, etc. are still unknown.
Figure 10 seconds for a mag change.
30rd mags = 3 mag changes = 30 seconds
10rd mags = 11 mag changes = 110 seconds
Difference = 80 seconds.
Doesn't seem like it should make much difference in the course of a 20 minute shooting spree.