Here's my 2 cents worth.
The new sup dvd mentioned on another thread ..(
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=31719 ).. says that the leash should be at least as long as the board.
So if your board is about 9' long it should be ok..

..It may also depend on if you like to nose ride.
Also you have the choice of ankle or knee position..The knee position type may need to be slightly longer and it help stop tripping on it..but it is not good if you like to kneel down paddle sometimes.
I also remember reading something about a new sup leash that's half straight and half coiled with the straight section dragging in the water and keeping tension on the rest...That sounds like a good idea.
DJ