How long until I get good at the sax or cello?

Personally, I would go with the sax. There's more avenues open to saxophone players, and especially good ones. If you have played the clarinet before, it should take you about three months to be semi-good and seven or eight to get good. Going to professional quality, however. . . maybe a few years, or a lifetime. Claude T. Smith workbooks will help you on your way. Just remember to learn vibrato either way. It makes a huge difference in the way you sound. Tips on how to get that "dirty" sound out of a sax: start by bending the notes and then move your tongue a little further back in your mouth.