Well I had a 40 point lead in my last cup game, & garbage time was triggered with 1:31 left in the 3rd quarter. There was a dead ball with over 2 minutes, but it wouldn't trigger then. But yeah, same thing with me a lot of games... rooting for a triple double but the player gets pulled.
Not sure what the sliding scale is though. It's still 20+ partway through the 4th quarter. Or at least it used to be. I've had a few games with an 18 point lead & 8:00 left in the 4th quarter. On a dead ball, my starters still stayed in...