You can currently do up to 8 of any combination of Battle and Gathering Guildleves, and seperately 8 Crafting Guildleves. I believe once you get to rank 25 and unlock the rank 30 leves they will offer more leves for you overall, 6 leves with 3 per camp. Until then, if you want to do more guildleves you will have to go to another city to get more, get lower level leves from the same city, or just wait out the guildleve timer. In the menu under Attributes there is a tab for Timers that will tell you how long it is until the leves reset, it's a global 36 hours like Coglin said.

Guildleves are getting revamped when the next patch hits, making them more flexible by giving us 4 slots every 12 hours and having those slots stack up to 99 over time, so you can do mass amount of leves whenever you have the time.

There are side quests you can do for XP that you should be able to find by talking to NPCs with ! bubbles over their heads. I think they're marked on the map so you can pick them out as you run by them. I'm not sure what level they become available but I think you should be able to find at least a few now.