This, as you can imagine, depends a lot on your involvement and the time you can dedicate to pronunciation.  Something I have noticed over the years is that, on average, it takes a week to pick up a new element when practicing every day. Another week to be able to produce it successfully, and two or three more weeks to reach spontaneous speech. Depending on the elements you need to improve, you can calculate the time you need. Also, the time you can spend practicing at home has a drastic influence.