In Japanese Particle suffix -kan is used to talk of about period of time but I have a question?

I am Japanese and I just know it, and can't explain why, but when you talk about weeks, you have to use 週間 weither you use で or not. The same for hours. You have to use 時間 for period of time. If you say ６時間, it's six hours, but if you say ６時, it's six O'clock. But when you use で, you don't always have to use kan for years. Hope it helps.