I'm assuming you're using a either a NURBS curve or a Bezier curve and not a Path?
If you go to the Editing (press F9) panel, in the Curve and Surface panel about half way down on the left is a value called "PathLen". Enter here the total number of frames you want the curve to last for.
There is also a speed IPO, I haven't played around with that too much but it looks great, you can control the speed an object moves along the path, so you can make it move quicker/slower at different times. Or, make the object move backwards along the curve.
Hope that helped.
P.S. Although the "PathLen" value is present for a Path, it doesn't seem to have any effect, I think this is because the Speed IPO is there and that is what controls it.