How often to rest ears while mixing?

People often say to take breaks from your mix to avoid ear fatigue and maintain accuracy. How frequent and how long do you guys prefer to step away from your mix? I.e. do you focus on getting one track right for an hour before taking a break then moving onto the next track? do you take care of the automation of all necessary tracks before taking a break then tackle EQ? etc

I find it helpful to take a short break every hour or so of mixing, no matter what aspect I'm focusing on. And after I've been mixing a number of hours, I can often tell when I've reached the point of diminishing returns, which signals that it's time to pack it in for the night. But it's not always that easy to know precisely when you've lost your "ears," so I find it really helpful to save my DAW files incrementally, with descriptive names for the files (e.g. "Vocal up 2 dB"). That way, when I listen the next morning and realize that I lost my objectivity a while before I finished mixing, I can backtrack to a point where it was sounding better.