Like the other guys said... 15 days???? huh? Gee I stretched my dick for ten seconds and it didn't look bigger.
Early gains are going to come from your suspensory ligament. If you do one exercise do Admirals. Before I even knew about
MOS i invented this routine for myself and gained 1.5".
Get an erection.
Hold your balls in your hand with your palm facing up.
Now move your hand back toward your ass so that your balls hang over the front of your hand and reach up to grab your shaft through your skin. ( for the record i have low hung balls so yours may not hang entirely over the front of your hand.)
Slowly pull your shaft down as far as you can. You should feel the stretch in the ligs that hold your cock against your abdomen.
Do this for however much you want each day. Don't tug sharply, use even pressure to extend into your stretch.
if you think you are doing it wrong, keep messing with your grip till you feel the stretch.
You should expect a 20% gain length in a little over a years time. It's like yoga for your dick. it hits the same ligs that surgery would cut for penis enlargment, but it's free and safe.
Good luck!