Microsoft says that mailbox provisioning usually takes less than half an hour but can take up to 24 hours in some cases.
In my experience it's generally at least ten to fifteen minutes, and an hour is not unusual. The unpredictability of the time frame is down to the fact that a shared platform along with many other companies, and someone else's activities could actually make yours take longer. Given that Microsoft is not highly motivated to overprovision their infrastructure to ensure maximum performance at all times, it's not really surprising.