There is no way to know for certain but if you want I can give a guesstimate (not the dev btw.) Looking at what is left to do, there is 5 Legends left to be completed with their own side events. The time to complete these mons may vary depending on the importance of the mon in question. From what I have seen if nothing important gets in the way, a mon and their human partner can be completed in 1 1/2 to 2 weeks, and that's just average mons the next update has Arceus so that could be like 3 weeks. Along with that however, there is the other batch of side scenes Chainsaw wants to add to past chapters. These may take a day or two due to needing to create proper images and writing the scene itself. Add ship scenes which secret spoiler, they already completed the milk scene, and I think late October early November if nothing comes up.