Depends on which bike I"m riding, and whether the winds are steady or gusty, and whether they're cross winds.

Around 30 I figure that even with my heavier bike and my heavy butt, there will be debris a-flyin'. Basically over 20 I'm likely to make it a relatively short ride, though there have been *fun* exceptions. There's nothing like going 20 miles into a stiff wind to really get you in shape, even if you end up resting three or four times in between... it's interval training au naturel!