This depends so much on the instrument, the learner, and the instruction! Some people are quick studies at any instrument; they pick it up and seem to know intuitively how to make it sing. The rest of us have to struggle on, hitting plateau after plateau (or is it wall after wall?). I practice sporadically, and the older I get the longer it seems to take to get even half-way decent at a new instrument. Practice, practice, practice and be brave enough to make mistakes when you're around other more experienced musicians- you'll learn so much from them! Good luck!