I've noticed one important thing about long mixing sessions (maybe I'll be the Cap here, but maybe not).
If I have to mix lots of stuff and the deadline is near, I'm listening the mix for some 20-30 seconds, then I write down what I hear is wrong. Then I fix it in silence. Then I listen again. Thus, actual listening (and getting used to a possibly faulty mix) is shortened greatly then listening and tweaking at the same time. So, for me it is (30 sec listening, 3-5 min fix)xN , then short rest, switch to another project, repeat.
Also, I'm switching between the projects after several improvements in each.
And the references are helpful when I start losing attention. I usually take the "unfixed previous mix" as one reference, "raw mix" as another, and a couple of my favorite records in addition. It really helps to "reboot" the feeling of balance.