Dark Prince: It takes 25 games as Vikings states to get a rated BKR.
Interesting you don't consider receiving a provisional BKR rating of 2500+ on just four games flawed. I know many on this site that would beg to differ, especially as one can then play and win 21 easy games and obtain an established (unrealistic) BKR.
There has been much debate on this subject in the brainking board.
SL-Mark: I already acknowledged the potential flaw in the established rating of those who avoid playing strong opponents. The term provisional already indicates a lack of accuracy. As such, a provisional rating can easily be igonored by potential opponents in favor of looking at the previous opponents in completed games or the games themselves to determine the players strength.