Each guild has a daily experience cap of 6,246k, and level 25 requires 845,670k. Therefore, the total number of days to get to level 25 is: 845,670,000/6,246,000 = ~135 days. Given that Cataclysm came out on 12/7/2010, the earliest any guild could progress to lvl 25 is 4/21/2011.
Edit: 100K quests completed in the guild, 100K gold looted, Armadillo is unlocked, 1st feast unlocked, guild page unlocked, still capping guild XP for the day in 3 hours tops with the change. Guild level 5 inc tomorrow and the coffers will begin to fill!
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"Shhhhh... do you smell that?"
So... Agent skully... funny running into you here