You have to consider two differt aspects: the volume of your monitors and the volume of you master bus.
- Your speakers volume shouldn’t be too high. Mixing at low volumes (not SO low! You have to hear what you’re doing right!?!?) won’t get your ears tired, it let us keep focus on the right choices during mixing, moreover it’s a blessing on mix drums: at low volumes we understand if kick and snare are in their right place and if they are present in the mix, it’s important!
- If your master bus have some peaks up to 0dB you’re making a mistake, first of all because if you reach 0dB you’re clipping your mix (very bad!), in addition if you don’t leave enough room on you master bus it will be really hard to put each track in its right place. Let’s say you’re mixing a song that will have to be mastered outside: you have to leave some room for the mastering engineer, otherwise it will be impossible for him to work starting from your mix.
Keep your volumes at reasonable level, don’t try to reach loudness as soon as possible, you’ll have just the illusion of a good mix but you’ll miss the point: what’s important during your mixes is EQ, compression and right panning, nothing else. Take care of those aspects then think about loudness, don’t just crank up your volume! 😉