Yo Man of HIT TUNES:
Just to give you one example, I've taken work from a vocalist backed by CD background music, easy to do in the studio and sounds rich. However, when I have the time, I will mix six or seven versions of the cut. I will change reverb rooms, adjust FX, pan knobs, change EQ freqs a little bit here and there, etc.
So, you can see that it is easy to spend several hours or days just working with one tune.
Imagine all the work when you have several different tracks of musicians to deal with? But, if it wasn't fun, you wouldn't be writing those HIT songs.
Have a nice Sunday,
Green Hornet