I did a couple 100% Brett C beers, didn't have an issue bottling after a month. Steady gravity readings are the best way to be sure it is ready though. My 100% Brett beers tend to carb up just about as fast as regular beers, but waiting 3 months you might neeed to wait a little longer.
Vinnie adds his “house” bug culture to Sanctification, which I suspect helps to prolong fermentation. He also uses Brett B and L which tend to be more aggressive than A/C. It also depends on pitching rates, aearation, nutrients, temperature etc… just like any other beer.
Hope the beer is worth the wait.