How long it takes to break in a high quality badger brush?

All depends on the batch of hair. Some are pleasant right away. Some take 15 lathers. Some hog lather early on, others do it constantly. Depends on knot density etc.

If you’re coming from synthetic knots you’ll certainly need to start loading more heavily

+1
It also may depend on the size of the knot; the denser the knot, the longer it may take, for some hair types.
As mentioned above, you may need to load more soap if you are using a badger brush.
What brushes had you been using prior?

I'll second these gentlemen's observations.  Hair characteristics, knot density, the amount of loft, even the way the knot is tied/glued--all come into play.  ("Quality," of course, is in the eye of the beholder.  So, I'm going to assume you're talking about one of the more expensive brushes.  Please correct me if I'm wrong.)  

With that understanding, some badger knots perform marvelously from the start while others need fifteen to as many as thirty shaves (not necessarily lathering's) before coming into their own.  I'll also argue that most high-end badgers--even those that perform well from the outset--tend to "improve" with use.  

All this is to suggest that patience can be a virtue. Your brush might never break-in to become one you enjoy (in which case, you might consider selling it). But on the other hand, you could have a gem just waiting to be revealed.

Thanks for your insight and counsel...

I have experienced from nearly no break in process to a shavemac d01 that is still breaking in after more than two years of monthly uses.