Win/Loss ratio required for promotion

[Thrutchy]

Here are some of my stats:

placement->clever: 4 wins, 1 loss
clever->gifted: 31 wins, 0 losses
gfted->master: ~25 wins, 1 loss
master->ST: ~33 wins, 5 losses
start->ST: 93 wins, 7 losses

Also, my experience is about 5 games after you get matched with the league above (and continue mostly winning), you'll get promoted.

Based on my experience and what others have said, I think it is about 30 games per league with a good win/loss ratio.