The initial time for the VM is 50 minutes but that can be extended multiple times.
In the upper right hand of the NDG browser page, there is a time counter. When the time goes under 15 minutes a message will appear to "Extend Reservation". However, if you have the VM undocked this will be covered up, so you won't see it. Make sure to leave that upper right-hand corner uncovered so you don't miss the option before the initial time runs out.