Guess it is partly down to personal preference, even in an hour session I would rather only come once (around the 45-50 minute mark). The second time never feels as good anyway so would rather just have a long build up, pop once then wind down for the last 10-15 minutes and be on my way.
But to answer OP's question, yea you should be able to do 2 or 3 rounds in half an hour if you can manage... after all you are paying for the time... this thread has been an eye opener though, I didn't realise so many guys could manage more than once in half an hour, I think I would struggle.