Hey bug,
if you ask me, 30 minutes is plenty, especially if you haven't been clamping for that long. Personally I've found that if I go beyond 30 minutes, my erections suffer horribly. But it all depends on how you feel: can you still get your dick up normally the next day after a girth workout? If you experience prolonged numbness or loss of sensation, then I'd suggest you stay at 30 minutes (or maybe even cut down to 2 sets of 10 mins) until your cock gets used to it.
Some light wet jelqing is probably a good idea for improved circulation, as well as giving your penis a massage of sorts. I usually do the jelqing before I clamp, for around 10-15 minutes, to "prepare" my dick for the intense work that lies ahead.
Anyway, it would be good to hear some more details, i.e. how long you've been clamping for and so on.