I can only comment from my own personal experience at long kegel holds, in that they cause no damage.
At worse you get the ache in the scrotum area from not shooting a load, but once done that discomfort eases.
As DLD put it, do these as often as possible. Don't even need to count, just do them, vary them with quick, progressive, slow, holds.
I don't think you can overdo it, some disagree, which is fine, I've done them for many hours at a time for nearly 12 months daily when in my hayday if you like, never had negative sides, just able to hold off ejaculation, and the my erection was like a lead pipe, the women LOVED it, much more then a big dick too.