After I cracked my ribs, I wasn't afraid of riding itself, but I was afraid of speed. Oddly, I wasn't even going that fast when I crashed, maybe 13-15 mph. I still get nervous when I get up around 30.

I was back on the bike at about 3 1/2-4 weeks (but I didn't have the pneumothorax). Just do what you feel comfortable with, and it will come.