If I already lost objectivity on a mix after multiple days or weeks on it, then even after a break I can get fatigued after 5 minutes, after 1 hour I have lost objectivity, and I am not listening to the mix for tone anymore, I am listening to how well everything is sitting, for which I can continue to do this for many hours with not too much of a problem.
With a reference though, I can carry on for several hours longer without getting anywhere near as fatigued.
Mix at a lower volume
I do a few things to allow me to work for longer,
Short bursts of playing the song and tweaking
Avoid Narrow boosts (sweeping method in an EQ)
Switching to different sets of speakers, and headphones periodically
Reference Mixes
Short Breaks
Closing my eyes
If you all of a sudden have the urge to do a massive EQ move after many hours of mixing, this is usually a mistake, this could be a good time to walk away. (make sure you got that backup save if not!)
Sometimes there is a glaring resonant problem in my mix that I just did not notice whatsoever until I listened on a specific speaker, and then once I notice it, the mix then becomes unbearable to listen to on every speaker. And it is never so simple to just narrow cut because it will either throw the balance of the rest of the track off, or just make another resonance pop out. Then...... I have come back to the same mix a month later, and everything sounds fine again. So I will wonder for hours..... is it just me?