I’ve heard it so many times from top artists, engineers, teachers and articles. What are your thoughts on this topic 
When enough is enough and you say to yourself it’s time to let go. You’ve done your best and it’s time to move on 
Some say the song will never sound like in your head and if it sounds good enough to you and amazing to others listeners, it is done 
A mix is done when I don’t know what else I can do to make it better, even though I know it could be better, just not how 
A mix is done when I don’t have any more time to put in to it due to a deadline.
A mix is done when I know I can’t possibly improve it.
A mix is done when I pass out on to the floor from complete exhaustion.
A mix is done when I can’t stand hearing the fekkin’ song anymore.
I don’t think that any of those scenarios qualifies as abandonment. 
Aloha guys,
I now use the ‘Square’ thingy with my phone to take payments from clients
so for me a mix is finished when the swipe goes thru.
{’-’}
Indeed!
It also ends (abruptly) when the client DOESN’T pay for services rendered thus far … if you know what I mean, and I think you do. Sometimes clients have to be abandoned … wait, “client” isn’t the right word … I mean “deadbeats”.
Write better music?
I don’t see it as abandonment.
A mix is done when I don’t have any more time to put in to it due to a deadline.
A mix is done when I know I can’t possibly improve it.
That and:
There comes a time when you listen and you just “feel fine”. Everything sits properly from an critical engineering standpoint and you are only left to judgment calls based on stylistic options which are viewed as personal taste. Those are decided by me and the band/client. Must-haves, yes, Maybe-haves bring budget considerations and overall thoughts of if it is really necessary.
Those above options are weighed by the composition as a whole. If the arrangement or melody is weak, poorly played, etc, that weighs in, but the arrangement and melody is usually optimized in the beginning stages.
I can’t believe you still have those emoticons from “the highway”. 
The emoticons at this forums are so * l a m e * so I had to!
You lob and I smash! Division of labour 
Awwww! 
But you’re the one getting tired of your mixes!
They sounds just fine to me. And long ago I heard some great guitaring as well! 
A mix is done when it sounds ok on my monitors, on my headphones, and in my '96 Honda Civic.
A mix is never finished if there is no clear understanding of what was recorded and why.
Good point one of my favourite band recorded and mix an album in 14 days. The lead vocalist said that they knew how to use the technology and knew exactly what they wanted