It really depends on the hill, the road conditions, and the bike. On a fairly straight descent I ride regularly, 40+ is no big deal. On another descent with bad pavement and a blind corner, I keep it closer to 30. If I didn't have to worry about oncoming traffic, I would push much higher than that. Way back when I first started riding in VA, the fastest I ever went was about 50. I never started getting nervous until I had to brake :-).