It's fine to have some sperm seep out of your penis when it feels good. It is called "pre-come" and it is perfectly normal. I get it all the time my g/f hates it lol! And if you have never masturbated before then it probably will take a lot less then 10-20 minutes because your penis isn't used to intense rubbing so it will be extra sensative. I'd say 5 minutes give or take a few.