Well. Judging by voyager at maximum warp about warp ten and they were 70,000 light years from home. And they said it would take 70 years. Then warp ten is 1000 times the speed of light. And warp 5 would be about 500 times the speed of light.
Hope this helps. =D
This is not correct. While it is true that warp 5 is in the magnitude of 500x c, warp 10 is an infinitely high velocity. See this diagram: http://en.wikipedia.org/wiki/File:Warptable.gif
Warp 5 on the old scale is 125 times the speed of light, and 189 times the speed of light on the new scale.