Personally, I don't think there's a "one size fits all" timeline for the CISSP.
It's a very hard test, but the "difficulty" level depends on one's own experience, familiarity with the industry, time available to study (while juggling work / home / family etc), and resources available to purchase study tools. Generally speaking >> If you have more time and money, it will be easier but not necessarily a slam dunk. If you want to save money, you will probably need more time but can definitely pass with a good plan and lots of time spent for preparation.
You can read about my experience if you'd like. I spent as little as I could and focused more on official resources and spending quite a bit of time studying on my own, but still managed to pass.