According to google, surgical repair of a fibula fracture can take 3-12 months recovery time. I'm not finding a lot of good comparisons for NBA players, as most of the ones that were big news (Paul George, Gordon Hayward) were tibia fractures not fibula fractures (fibula is less severe than tibia).
I'm currently hoping he can start rehab by the end of the season and be ready to go by early next season. That's probably best case scenario. Unfortunately he'll very likely miss having a normal off season. Worst case scenario... well, I'm not going to put that out in to the universe.