My method is simple, I count to ten, ten times with one hand, and then ten to ten ten times with the other hand, and do this twice (for a total of 400).
Example
1,2,3,4,5,6,7,8,9,1
1,2,3,4,5,6,7,8,9,2
…
1,2,3,4,5,6,7,8,9,10
When I first started, my wrists would hurt and I would have to alternate at the 50 mark, but you get the general idea.
I figure reps is better than time, because you’re getting a consistent workout, this could also prove useful in the future if the gains really do start to show (and so far I’ve been getting a lot better erections, but haven’t measured or anything yet). Going by time could mean on a tired day, you do half the workout you might want. Going by reps forces you to reach your goal.