if you joined late 2004-2005 = about 1yr to upgrade..
Therafter from Early 2005-late 2005 = 1.5 - 2 yrs
Early 2006 - till late 2006 = round about the same, 2 yrs plus/minus few months
Therafter... They are slowing down, the time should circa. 3-4 yrs, that's only an educated guesstimate!!
No credit I'm affraid for previous command time.. purely Seniority!!