Little over a hundred hours. Have beaten the Heart on A20 with all three base classes. Like you, I find Silent to be the easiest of the three in high ascension due to the options, and the hardcore streamer win rates back up that claim. Defect is a little wonky, relative to Ironclad. If you get a winning build, it's cake, but assembling one isn't the easiest thing in the world to do.
I could run Watcher up to A20 easily enough, but I find it not to be fun. There are a few stupid combos that just win on the spot for that class, and it's not all that hard to play into them. Deliberately not taking cards to keep certain infinite possibilities alive just doesn't feel much like StS for me, and it gets repetitive quickly. It's like hard resetting Isaac for just Knife, Ipecac, Epic Fetus or Dr. Fetus, the way a speedrunner would. Yeah, you'll win, but it's also the same thing over and over again and gets tiresome quickly.