It is very hard to estimate, in particular with this series which has completely shaken up the status quo from the previous series. Because the characters aren't tied down to any particular region (or any particular set of games) and can basically go anywhere, writers have the freedom to do whatever they want for as long as they want. There are no gym quest or trials for Ash, only his participation in PWC, which is also super unpredictable, because the ranking system allows the writer to prolong this quest for however long they want to. Goh's goal is to catch all Pokemon, which is something that will take either a long time or forever, so he could also stick around for a very long time.
I would imagine if they follow the previous series, somewhere about 150 episodes, maybe? Again, it's very hard to tell, especially with how much the structure has changed.
Theoretically, gen 9 games could come out, and the writers could technically just keep following Ash and Goh's adventures, and the only obligation they'd have is to have them visit the new region from time to time, or set a storyline there.