How Long on average to master a track?

You really can't. That is a good read on exactly what Mastering is and why it is important to have done for your music. Mastering is prep for duplication, which these days means little seeing as how we don't need master copies because we are all digital. Still, mastering engineers clean up after mixing engineers, and give a fresh and unbiased ear to your album as a whole.

Here is a before and after. Mix by me, master by Dave Collins. Mix: Master: I have about a zillion other examples if you want. Omni, you around? Care if I post an OFF tune as an example?