When is a chapter too long? How many pages in a word document is too long for a reader? Or is there such a thing as too long?

YES keep it at 5,000 MAXIMUM most people dont like reading huge chapters

2104393 The best FiM fiction out there Fallout: Equestria has a chapter over 50 000 words long and nopony complained. The chapters should be as long as they need to be.

But yes, usually chapters are around 2k to 4k long.