It all depends on how you define, "mixing." I spend forever on tunes so maybe I'm not the best person to give advice. Separating the two stages is often recommended. Personally, there isn't much difference between them for me anymore. It's turned into one interactive process. I often go back and change things in the mix during mastering. Then I use mastering just to take that little bit further toward the sound I'm after. It's often about pushing things a little too far and then pulling back just a little bit.