The answer, of course, is a resounding “it depends.” If you are talking about a live situation, then the answer is somewhere around “whatever is appropriate for the genre of music you are mixing.” You wouldn’t mix a traditional bluegrass band at 120db, and likewise, a heavy metal act at 70db would be pretty silly.
If you are talking about mixing in a studio, then the answer is a little different. There is no “one” answer, for several reasons.
First though, consider “Fletcher Munson.” In essence, what this means is that your ears have different sensitivities at different volumes – i.e. your ear’s frequency response changes with volume. At lower volume, your ear is most sensitive to frequencies which coincidentally are those which add intelligibility to speech (2KHz – 5KHz), and at low frequencies, your ear is not very sensitive at all. At high volume, your ear is much more sensitive to low frequencies, and to a somewhat lesser degree, the high frequencies. So, at 60dB, a mix will sound thin, with no low end. Turn that same mix up to 110dB, and it will have a ton of low end and much more high frequencies.
If you are thinking that there must be a volume range somewhere in the middle where the ear has a fairly “flat” frequency response, you would be right. And that mid-level volume, that is neither too loud nor too quiet, where your ear has the flatest frequency response is around 80dB. If you get your sounds and do your initial rough mixing at 80dB, then your mix will have a frequency response that should be fairly representative. If you get your sounds at 65dB, then when you turn your mix up, you will have WAYYYY too much low end! Likewise, if you start out at 110dB, when you turn it down, your mix is going to be thin and lifeless (sounds like a commercial for a hair care product, doesn’t it?).
However, every once in while, you need to vary the volume you are mixing at, again for a couple of reasons. One is to double check your mix at different volumes, but the other is because the ear develops “ear fatigue” when listening to something at the same level for long periods of time. Never listen for too long at any one volume – it will all start to sound the same, because your ear is developing fatigue. In fact, you should completely take a break fairly often. Leave the control room, walk outside (unless the sun is shining!), get a drink, play a video game. Just do something different for a while. Then, when you return to your mix, you will “have a fresh set of ears on.”
Along with checking your mix at different volume levels as you go, you also need to check it on different speakers. Try some near-fields for a while. If you have a different set of monitors, use those for a while. Some people even have an average (i.e.-crappy) car speaker, and will listen to a mono mix on that. Play it on a system in another room. All this is to verify that the mix holds up well on a variety of speakers, at different volumes, and in different rooms. Sometimes it will sound pretty good in every situation, but sometimes you will hear something on another system that just sounds somehow wrong. Go back to the mix, fix that problem, and try again.
Don’t drink alcohol before mixing!!! Alcohol does horrible things to your hearing, PLUS is makes you much more susceptible to hearing loss. The same thing holds true to a lesser degree with caffeine, but there are limits to what you need to put yourself through to mix, and caffeine deprivation is beyond that limit. Also, lack of sleep can affect your hearing.
Notice that I have said nothing about what the band has to say while all of this is going on. There is a very good reason for that – it’s a whole ‘nuther can o’ worms which would take many more paragraphs to go into.