How long you study for this exam is relative to your own individual study habits, current knowledge, and ability to translate those facts into real-life situations.
I studied intensely for 3 months for the ACE Health Coach exam. The exam is difficult, so I would over-prepare for it. There is very little regurgitation of data—so be prepared to put yourself in practical real-life situations with the information you learn. Use all of your study materials, take any practice tests, and even make flash cards. Also, although the exam was multiple choice, many of the possible choices came down to 2 possible good answers when I took it. That made the exam more challenging.
Good luck to you,