Giessen offers a lot of different locations to party hard. Clubs, bars and discos, everything is really nearby. The most locations are in the city center in the pedestrian area or near the main station. Another good place to have fun is the Ludwigstrasse. It starts at Berliner Platz and leads to the Otto-Eger-Heim, a residence of the university. There you have many clubs and bars, one right next to another. At least once you should try to get the Ludwigstrassenschein, a card you get when you drink a shot and a beer in every bar and club. Quite a challenge! A few big clubs are a bit outside the city in the Automeile area. You can take a bus there from the city center.
Partying in Giessen starts pretty early, almost right after classes. So the next day you can go to you classes again. At about 23pm is the perfect time to get in depending a bit on where you go. Clubs and discos cost entrance normally not more than 5€. They close at about 4am. Bars are mostly open almost all day because they offer snacks as well and close a bit earlier than clubs at about 2am.