One of the keys with kegels is not so much about how long you can hold each one...but more so about how many you can do. It's a repetitive exercise, meaning you're supposed to do MANY in repetitions. I'd suggest starting with a couple hundred a day throughout the day whenever you're able to sit for a few minutes here and there to do them, and whenever you think to. Progress up to 500+ daily. You'll definitely notice an increase in things like stamina, erection strength, ejaculation distance...possibly in ejaculation volume too, as some men report.
You can also do towel raises for even more benefits with kegels. See the home page
Advanced Exercises - Penis Enlargement Gym for details on those.