Hey Krespe, the maximum brackets that Guilds can climb isn’t currently publicly available, however if your Guild is very active in Guild Wars and scores well against your opponents you will move up fast.
This may or may not be as fast as you’d like but given all the time our top Guilds took to reach the top brackets we feel this is fair and certainly a lot faster than the old system when Guilds could only move a maximum of 2 brackets per month.
Good luck!