Gliders can fly for as long as there is lift, typically in the UK (at Lasham), this will mean using thermals. The best gliding days will allow for a glider to stay in the air from 9am through until sunset. The record from Lasham is Durham Cathedral and back.
When in training you will typically fly for between 10 minutes (no thermals) and 1 hour. While training you will either get 1 long flight or 3 short flights.