My answer to the OP is one foot for every 10 mph. That works. Descending, I tell people to space themselves 20'-40' apart. There's no reason to get close when your doing 40-50 mph.
These drafting threads sure get out of hand here, don't they? One thing which is very noticeable is that no one ever tells about having an accident because someone was drafting them - other than in the team TT video posted above. It was fairly obvious that the lead rider wasn't looking where she was going, rather more interested in how her team was doing. IMO. The thing to pay attention to, whether someone's drafting you or not, is what's in front of you, not what's behind you, duh. I've been on over 1000 group rides and have had countless riders draft me without asking or notifying and never, that's never, had a bad experience from it. I've had 30-40 people behind me, no problems.
One time my wife and I were heading north on Hwy. 1 in California, riding our tandem in an event. It was blowing the usual 25 from the north. There'd been a long climb and descent recently with quite a few people passing us on the climb. We were overtaking these fast folk on their singles, holding a steady 9.3 mph. One by one, we'd pass them, and they'd hook on behind the last rider. Hey there oh negative ones, we were doing a service for humanity! The pale exhausted woman with the crushed soul, 30' behind her a-hole man, she shouldn't have gotten a tow from us? Where are these posters' hearts, not to mention their heads?
What it is, is this thing, I believe an uniquely US thing, with a few who are always hating on people who get anything for free, even if it's only the fart-filled wind of our passing.
What happened to the idea, quite popular on BF, that the fastest way to get fit is to ride with people who are faster than you? Where'd that go? How does that work if no one can draft? Hundreds of riders have jumped on my wheel, and I've returned the favor - after all the second rider does allow the first rider to go a hair faster. Riding with others is one of the joys of cycling, certainly nothing to b--ch about.
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