You just can't convert distance in time without a speed factor.
For instance, for the grenade spell I use, let's say X is the caster and Y the target. The formula I use is (Distance between X and Y) / (Speed of the missile)
Since W3 uses the same unit for distance and speed, if I try to cast a grenade at 2000 range with 650 missile speed, it will take 2000/650 = roughly 3 seconds for the grenade to hit the target point. And so far it works =)