Audio mixing is one of those tasks that I think works best when done spread over several sessions.
The reason is fairly simple, to help keep perspective on the track. Working too long causes your ears to become tired and before you know it that tiredness skews your mix. Apart from that it's good to leave a track and come back to it to see if the ideas you have been working on truly are working out.
When working on mixes I regularly make a mixdown that i can play on a number of systems, checking that it sounds pretty good whether you listen on an ipod or a car stereo or your home hi-fi. It's amazing how much this simple process of listening on different systems can reveal. The differing frequency responses of the systems and the differing listening environments have an effect, coloring your mix and highlighting potential issues.
Closing the loop you then return to your mix at some later point with fresh ears and fresh perspective ready armed with the lessons learned from listening to your mix on a variety of audio systems.
The Home Recording Studio
More and more now I work on arrangement as I mix. That's the flexibility of having your own recording system. I dread to think about how much it would cost to do something comparable in a professional studio. True it has cost me up front to set up a decent home recording system (and believe me there is still much room for improvement!) but the overall benefit is the luxury of being able to spend time experimenting with recording, mixing and arrangement. Sometimes I write a song and at the same time record it, which for me is definitely a luxury worth paying for. Especially as it is actually cheaper in the long run.
The benefit to my mixes and to the song over all is, in my eyes / ears, like night and day. Creatively it is far more fulfilling and the finished product is far more likely to make it as a "keeper".
Songstuff author Cheryl Hodge captures the evolution from recording studio through home studio towards a professional home studio very well in her article "Protools And Me (The Singer / Songwriter)"
If you write your own songs I can honestly say that the investment of time and money in setting up a professional standard home recording studio is well worth the effort. Go on, benefit your songs and invest in some gear!