In theory:
20x = 3 minutes
18x = 3 minutes, 20 seconds
16x = 3 minutes, 45 seconds
12x = 5 minutes
10x = 6 minutes
8x = 7 minutes, 30 seconds
6x = 10 minutes
4x = 15 minutes
2.4x = 25 minutes
Of course, this doesn't include formatting and closing the disc, so add a minute or two to account for that.