The urban legend is that L. Kravitz must hear his mix 20 times in a row without wanting to change anything in order for him to feel that it's done. An hour and ten minutes, on average! Digital Domain have mentioned spending sometimes up to an hour or so on the first song or two of an album, while getting the chain established and the general séance going... After that, it can go more quickly, provided the material permits. Half an hour is the average, but sometimes a 15-minute quickie gets through, yes? Can't transfer a 3.5 min song in 1 minute unless you didn't listen even one time to the whole program.
Indeed, one doesn't want to get used to a bad sound - either from the source, itself, or from the processink one is doink to source during premastering session. To avoid this, rather than limit the amount of passes I do to the song, since there may well be clicks or surprises (rotten Easter eggs?), I keep reference material in rotation on a path of the detangler. At the push of the button, without moving gain knob for monitors, one keeps in touch with the real world. But one also doesn't want to miss something that would be heard by a fan who listens repeatedly to the track and learns its foibles better than the premastering clerk...
Fortunately, you are working in a different room than that in which the mixist worked, and, therefore, your acoustics are wreaking different havoc (at least microhavoc?) on the neutral (potential) playback. So, even if you work on the song for an hour, and thereby got ears used to a somewhat skewed treatment, you are still in a better position than the mixist to normalize the balance of the spectra. They pulled this way - you pulled that. Now it sounds like's balanced flat. (: (if only...
Make your room sound really good when playing really good recordings and then have fun making others' work sound its best. Don't worry too much. As Calbi's signature goes, Enjoy Music!