Came across these the other day and thought others might find them handy. These figures are not exact, just reasonable approximations rounded for ease of use.
One day of travel at Warp 1 roughly equals:
2.5 hours at Warp 2
30 minutes at Warp 3
15 minutes at Warp 4
7 minutes at Warp 5
3.5 minutes at Warp 6
2 minutes at Warp 7
1.5 minutes at Warp 8
1 minute at Warp 9
45 seconds at Maximum Warp