Leves reset every 36 hours (not 48). You can figure out when the next leve reset is by typing /clock into your chat window.

If you've finished your leves, there are other things to do. You can do behests, sidequests, main story missions, job missions, grind, craft, explore, or socialize.

The guild leve cards show the objective. But it's farther down the card, rather than being at the top. Once you find it on one card, it's in the same place on all cards. It will say something like "Destroy the monsters from the Necrologos" or "Kill 3 aldgoats."

Have fun!