I'm only a month in to Penis Enlargement, but I know when I first started I could only stretch for like 7-10 seconds before losing the grip ... my hand would slip and I would end up pinching/pressuring the head ALOT. After just 1 month I've noticed the grip and stretch ability is a whole lot better.
I am certainly not a newbie with stretching , I try to do at least 40 reps of 30 second stretches , plus a morning routine with a bit more than half of this. This is combined of A stretches , as well as the usual up, down, left, right. If your only starting out I would suggest about 15-20 stretches per day, and gradually add more in as you go.
30 seconds is a good minimum stretch time. As you get stronger you can mess with longer stretches . I have one stretch routine I used to do before I started hanging where I would hold a stretch for 20 minutes by switching hands as I got tired. Now must of my stretches are based around 60-100 second stretches ...some days if I am tired I may go with as little as 30 seconds.