I've often heard this but I'm not sure quite how that works out.

Some typical figures seem to be around a 100-150A current draw for a CAR starter motor.
Drawing 12V and assuming you run the starter for 5 seconds (I probably run it for 2 max) you have 12*150 = 1800W and 9000J of energy used over the 5 seconds

Assuming you have an excess capacity of 50W from the charging system (must do as I'm running a 100W bulb and heated grips which draw up to 70+W over stock with no problems) that would take 9000J / 50W = 180 seconds to replace the lost charge. Even with only 25W spare you'd replace the charge in 6 minutes.

From what I've seen in Guernsey where most journeys are 5 miles or less you'd expect EVERYONE to be hooking their bike up to an optimate if 20 mins was the requirement (especialy since top speed is 35mph so the bikes won't be reving that hard) but that's certainly not the case from what I ever noticed.

I guess a loose rule of thumb would be you need to ride the bike for as many minutes as you had the starter motor running in seconds (which assumes 30W spare electrical capacity). eg Really bad start in the morning running the starter for 20 seconds requires 20 minutes of riding.