I would never bother with a 15 minute punt. I like a slow build up!
Half an hour is good for a girl I have not seen before to test the waters.
My preference is for 1 hour. Even though I usually pop at the 45 minute mark, I would rather do that and spend the last 15 minutes winding down; with half hour punts the whole thing can feel a little rushed and I often don't feel that I have time to do things like RO, which I enjoy.
There are some girls who do 45 minutes appointments, but they are rare.
If my punting budget ever halved, for whatever reason, I would be happy to exclusively do half hour punts. Still plenty of time to get sucked off and fuck a girl in a few different positions