I'd never come across the "10 second rule" before. If I have understood correctly, if, as you pass a slower vehicle and think about returning to the inside lane, you see that there is another slower vehicle ahead such that you would be in the inside lane for less than 10 seconds before you would want to move out again in order to pass that vehicle, then you are probably better off staying in lane 2. A rule of thumb. I like it and will try putting it into practice next time I drive on a motorway.
Note that I see it as a rule of thumb. If I'm being tailgated in the second lane of two, then I'm better moving over. If the car behind me is still getting off the ferry at Dover as I pass Gretna Green then 15 seconds in the middle lane of 3 isn't doing anyone any harm.
(Note that my irritation in my previous post was of the "grumpy old lady muttering into her cornflakes" kind. I reserve full on, give me my machine gun now, road rage for people who endanger my life when I'm riding my bicycle...

I do think that a desire not to be thought to be hogging the middle lane is one of the reasons people don't leave enough space in front of the vehicle they've just passed. I try to start indicating left the moment I get past the slower vehicle, but only move left when I think I've left enough space.)