Well, that depends on how careful you are.
Using the BMBF and following all of the "rules" I would imagine you could keep a beer for months, or longer, and be fine. However, I admit that sometimes I half-arse it and I don't chill my bottles and I don't release all of the pressure etc. The reason for this is because when I use the BMBF I am typically just running to a buddy's house for a couple brews. However, I did this one time and never got around to leaving so I had a couple of these in the fridge for a couple of weeks. Went over to my brothers house with them one day and they were wet cardboard city.