When my low end sits heavy and tight on the mix.

 

A mix is never really finished - ever. You decide it is good enough/sounding great/can't take anymore and that's it. If you don't, you can pretty much guarantee that the very next day you will listen to your "finished" mix, make a small change here, a little extra something there...and off you go again.

Knowing when it is ready and time to put it to bed is mostly down to the harsh reality of deadlines, I'd say. Obviously, things like: it works at different volumes, on different systems etc are essential, but the rest of it is so subjective and changes every hour of the day.

 

when i mix down i feel its ready when i start to hear that cd and radio feel. once i hear that then i say ok its done lets move on to the next project.

 

this is it exactly

 

do not release/show a song over-hastily, if you not 100% sure that all sounds that you want are included. let the project rest for some days. keep a bit distance and begin with a new one, maybe you will drop better sounds, which could also fit well in the previous project. try it!