The length of the flight depends on weather, type of glider, launch method, and pilot skill. On good days it is not uncommon for glider pilots to fly 100-300km and stay in the air for many hours. In the UK 1,000 km flights are sometimes performed, while the world record is now over 3,000 km.
While you are learning, most of your flight will be 5 -15 minutes long as you will focus on learning the take-off and landing before progressing to cross-country flying.