10 seconds. If you're not overtaking anything within 10 seconds, you should pull into the inside lane.

This is what I was taught in advanced driver training. However, in practice I use 7 seconds, because I find 10 seconds a little too far to easily estimate by eye. I end up thinking "is that 10 seconds? I'm not sure, maybe" then by the time I figure out it was more than 10 seconds it is now less than 7.