Yeah, numb is a sign you have gone too long.
With cheaper introductory clit pumps, both an advantage and a disadvantage (but, I think for beginners, more of an advantage!) is that the bulb-style pump doesn't allow for more than a medium-level suction and loses air with time. It's a nice safeguard against pumping too hard. Also, using lube will both help make a seal and protect the skin from injury. As the salesperson who sold me my pump said, "you don't want stretch marks on your clit."
I usually go thirty seconds to one minute, rest for at least the same amount of time, then pump again. I do a repetition of three to five pump-hold-relaxes.
