I usually wait until I'm done with the first draft to figure that out. With my current WIP I mark how long each chapter was. So far I've noticed a trend. My chapters are either near 3,000 or 6,000, so when I redo it, I think I'll try to make each chapter around 3,000. Of course there are outliers as of now such as my nearly 7,000 word chapter and my 1,700 word chapter. I'll have to cut and reattach if I want this to work.
To me, though it doesn't matter how long chapters are. In my book currently being beta read, I have four page chapters and I have 20 page chapters. I kept it that way because there was 2 POV's and one tended to have short chapters (but more of them) and one tended to have long (but fewer) chapters.