I'd go with 3mm as a reasonable minimum clearance.

As others have pointed out, there's no hard and fast rule. If it fits it works, but if it's very tight, you have to worry about getting home with a bent wheel, broken spoke, etc.

On second though, perhaps people don't worry about getting home with broken spokes anymore now that there's cell phones. Before cell phones wheels had 36 spokes. While I've gone off tangent a bit, the point I'm getting to is that you can push the clearance thing a bit more if you are running 36 spokes rather than say 20.