Just recently finished a 10 gallon batch of Brett primaried pale beer (95%belgian pils, 5% wheat malt) using a 5 day, 2L starter of Brett lambicus and Brett bruxellensis combined. They flavor is awsome so far. I let the entire 10 gallons (split into two 6 gallon carboys) primary for 21 days.
After 21 days of primary, into one carboy I simply racked 5 gallons and will be letting it secondary for 21 more days.
Into another carboy, I racked the other 5 gallons onto 3.75 lbs of fresh raspberries just picked up from a farmers market outside of Santa Cruz (frozen and then thawed), and added Pedio from Wyeast strait from the package. This one will likely sit for another 3-6 month in secondary, depending upon how fast the flavors develop.
Personally, for just Brett, I think 3 week primary and 3 week secondary is more than sufficient if you have a healthy population of yeast to start with and are within the temperature range for the yeast. That has certainly been my experience.
Hope this turns out well for you! Good luck!