It depends on the grade, visibility, road surface, curves. I can think of a hill with a 3-4% gradient, no curves, good visibility and decent surface where I consitently get as aero as I can to eke out that last 1-2mph. I usually hit 33; best ever was probably 40 with a tailwind.

I can think of another hill with a 15% gradient where the road is straight but visibility isn't so good (trees). I'm on the brakes every time I see the speedo go much over 30 -- 30 turns into 50 awfully quickly on something like that (as learned on my first trip down). A stud-muffin I used to know claimed he hit hit 60 on that hill on his TT bike. I can believe it.