There is much more time to be spent in recording and mixing, than there is in mastering. However, IMO, it takes a skilled, second ear, to do a mastering properly, and truly give a well mixed project the treatment it deserves. I have mixed and mastered many projects, but I can honestly say, that it really is best to have a trusted mastering pro, take over if you can afford him.
That being said, there are many tools that you can use, to do the job yourself. I end up mastering my clients projects 90% of the time, due to budget constraints of the band. The problem there, lies in the fact that it is tough to actually listen objectively to a mix you have heard over and over while mixing.
Once, I accidentally lost a cello track, because I just heard it in my head, from hearing the song so many times. It was muted. We tend to lose perspective, when involved so deeply in a project. At the very least, take a week or two after mixing, before you attempt a self mastering.
Just my opinion, take it for that...