Depends how fast the fights start.
The flashback still has another 3-5 chapters, most likely. I think it'll be another 20 odd at least for setting up the fights- especially since Oda hasn't even introduced the remaining Tobi Roko and the Numbers. Based on his previous chapters, it'll then be about 5 before the first fight is won.
The fights will more than likely follow the pattern of 1 chapter for the lesser opponents (Tobi Roko), 2 for the Calamities, about ten for Luffy vs Kaido. The most conservative estimate here is 20 chapters purely to finish the fights. Oda might treat us and give each Strawhat a two chapter chunk like he did in EL, but given his recent statements I find that unlikely.
Then a rather large post fight chunk of story- probably of a similar length to the aftermath of Dressrosa and EL, so 10ish chapters.
So my guess would be ~60 chapters, putting the finish round about next... summer? I think.