Yeah I know, same here.
In Virtua Fighter 4 EVO, you'd have to win about 4 or 5 matches straight in a row and you get a rank up.
In Tekken 5, --it's usually a streak of 10 or so wins. So, I'd say --as a guess that is-- you have to win 10 in a row to rank up.
...but does it really matter?
You know, you still obtain gold by winning the fights regardless of rank, and you still buy items for it.
So who cares if the thing on the top left says 1st Dan as opposed to Master. Same game.