You should try for up to 50- 5 second holds throughout the day, but remember to start off slowly! This is NOT an area that you want sore from fatigue!
Another variation of this exercise is the super-kegel. It's performed the same way as the regular kegel, except you try to go for holding endurance instead of repetition quantity. Try to work up to 5- 100 second holds spread out throughout the day.
They can also be done at the end of your PE routine to drive some much needed blood to your penis.