Welcome to SC!
When you buy a sub (or download the free ones), you’ll get a PDF alongside it.
That PDF is all you need to know to start. There are other reading materials but the PDF is the most important. Follow it to the T the first few months you use the sub. Then adjust it lower to your liking.
Then there is microlooping, extra rest days in the middle of the sub, long washouts etc.
All of these boil down to listening less than the recommendation in the PDF.
The goal is to get the recon (the process of reconciling with what the sub gives you can sometimes bring up repressed emotions, lethargy etc, we call this a “recon”) to be as little/manageable as possible so you can perform at your best.
If it’s indeed more powerful, we’d have recommended it too.
But even the usual every other day for 21 days schedule is too much for some.
21 days of listening every other day + 5 rest days is the max.
Don’t exceed 21 days & don’t decrease the long rest days at the end of the cycle.
15 minutes might seem too short, but trust me it’s hard to consistently hit 15 minutes with some subs.
The current recommendation even suggests starting from as short as 30 seconds.
You’ll understand why the more you use subs from here.