On https://www.torproject.org/about/overview a period of about 10 minutes is mentioned:
For efficiency, the Tor software uses the same circuit for connections that happen within the same ten minutes or so. Later requests are given a new circuit, to keep people from linking your earlier actions to the new ones.
Using another circuit for every connection or even page request (HTTP allows reusing connections) would be really slow and you would have used all available exit nodes after some pages. So this is probably not a good idea. Why would you like to do that?
While I doubt there is an easy way to switch circuits for every request it might be possible to lower the time a circuit is being used. Settings like
NewCircuitPeriod are probably useful in this context but since I never had the need to change the behavior, I don't know what else is needed.