Tor will reuse the same circuit for new TCP streams for 10 minutes, as long as the circuit is working fine.
(If the circuit fails, Tor will switch to a new circuit immediately.)
But note that a single TCP stream (e.g. a long IRC connection) will stay on the same circuit forever.
We don't rotate individual streams from one circuit to the next.
Otherwise, an adversary with a partial view of the network would be given many chances over time to link you to your destination, rather than just one chance.