Is there a threshold on distance/size for a tidal locking? I know that some systems tend to tidal locking (such as earth-moon), which occurs basically because the difference in the gravitational pull on one side is significantly different from the pull on the opposite side. (There's a lot of great answers here.) But what can be considered a significant different?
In other words, is there a threshold on distance or size in order for systems to tend to tidal lock?

Thanks!
You can see an example of tidal locking and atmosphere simulation for a planet closely orbiting a dim star. They show a simulation of the atmosphere and some interesting theories about the movement of gasses due to tidal locking (convection) that occurs between the bright and dark side of the planet.  The link goes directly to the discussion of tidal locking:

KEPLER 186F - LIFE AFTER EARTH

A similar question was posted in the physics forum and it looks like that could be close to the answer you are looking for:

Tidal Lock Radius in Habitable Zones

I am also interested in tidal locking and hope someone may post a more concise answer.