Never!
If a band want to sit in on the entire mix they can find someone else to do it. But that's me. I have enough work as it is so to take on a job like that I can allow to have it on my terms.
Now, in an atempt to be fair, I try to include the artist as much as I think is neccesary for their egos and mine, but I find that most often they can't be bothered to sit in on the mix anyway - unless they are newbies, but they find cheaper alternatives than me, usually. I do, however, let one or more band memebers sit in on a 3-4 hour rough mix session just to set things up at first. If they don't want to do that, I do the rough mix on my own and send them a high resolution mp3 to get some quick feedback. Usually they they say: "sounds great, but more cowbell, please". Then I mix according to what their initial feedback is and add in special instructions - like "delay on the lead guitar in the chorus of song X1, phaser on the drums on the intro of song x2 and autotune the bad singing on X3". Then I have (as few as possible of) them in the studio for a mix review and final adjustments. But if they can't be present for that I send them the mix on every song and receive a detailed list of changes in return. Then I implement the changes and send them the new mixes. Depending on their budget, this final process is repeated a few times but that doesn't happen very often any more. On very rare occasions I am working with someone who prove them selves to be an asset in a mix situation, who understand the process, when to shut up and when to contribute. That way we can bounce ideas off each other in the process which can be both fun and educational.
I think that this is the way things are done these days. I work with people all over the world, as a sound person and as a musician, with music and with film/versioning, and it seems to be the standard procedure.
If you don't want people to hang around in the studio when mixing, let them know that the price for the mix can easily double because of the distraction and as long as you do a good job with their music, they never really complain about saving the extra money