I sat down to try and write an End Rant describing how to know when a mix is done, and then I became stuck. Everyone's experience with this can be different, so I dropped a line to some colleagues to see what others had to say. -LC
When you can listen all the way through and there's nothing you want to change.
Sometimes my close-listening, hyper-focusing brain latches onto an "issue" that might actually be one of the elements making the mix interesting and special. It's a big time-suck trying to kill what isn't broken, and often it can make the mix worse! So, if I can check out the track(s) while distracted with something else – laundry, a jog around my neighborhood – and nothing jumps out to me as a problem, it's a good sign that it's time to stop twiddling knobs.
A mix is done when I remember it sounding better five mixes ago.
It's a good idea to save mixes regularly, even at the beginning – sometimes you don't know that it's done until it gets overcooked/overmixed and it starts to topple out of balance. That said, sometimes other interesting peaks follow when you keep going. Once you've had the benefit of going through the meticulous details, try also doing a fast, instinctive, emotional, gestural mix.
I send mixes off when I can't find something that I believe needs work, or if I know it's almost there but I can't figure out the last bit. Either way, the client, with fresh ears, will help with that last bit and make it really sing.
As most will say, a mix is never really finished, it's just released. But I find that at some point I just run out of things to do. When I start to second guess myself, it's time to stop.
When the song's climax is clear and apparent to most listeners.
The mix is done when I listen back and only hear a song and not a bunch of problems to fix!