Make them based on in-game time, but with a cooldown (say 1 hour, adjust to taste) after log-out during which the boost's timer is still running.

It's a way to avoid people jumping in and out to try and game the boost system to its maximum while also preventing people from losing their entire boost if the server crashes / they have to sleep / etc.

This system was roughly inspired by a system from another game I play, Puzzle Pirates - the "Badges" which unlock certain activities are time-based but don't deplete if you're logged off for a while.

Option 2 (because someone in the comments said my first possibility removes the incentive to keep playing): Boosts deplete at a slower rate while logged out (say, 1/4 of the speed). This incentivizes you to keep playing, but prevents you from getting punished quite as harshly by a disconnect or logout.

Thoughts / opinions? Boosts right now are kinda lame because of the way they work but if they were actually properly time based I could see myself buying one for next weekend.