We can compare the following release date to get an idea :

vBulletin 6 : 2016? + 4 years
vBulletin 5 : 25 sept 2012 + 2 years
vBulletin 4 : 22 fev 2010 + 6 years
vBulletin 3 : 20 jan 2004 + 3 years
vBulletin 2 : 4 jun 2001 +1 year
vBulletin 1.1 : apr 2000 (first public)

As vBulletin 5 was a marketing failure, vBulletin® Version 6 could be released soon.