I've been thinking about how to express a duration of time and how in the past I have added 間{かん} in situations where I found that I shouldn't have.
Take the following sentences as examples.
○: 30分後ご飯を食べる。
☓: 30分間後ご飯を食べる
○: 3年前日本に行った。
☓: 3年間前日本に行った。
○: 3時間後出かける。
☓: 3時後出かける。
Of course there are other examples that I could give but I think this kind of boils it down to the main point. When I think of 間 as a concept I generally think of it as an emphasis on start-to-finish time, like 1年間日本語コース or something like that. Why, then, does this not apply to expressing time in the future/past? Specifically I have said before something like 1年間後日本に行く or something like that, using 年間 in the same way that you would use 時間, only later to find that is not natural usage.
So this leads to the question of why these words cannot all be used in a consistent way. Is there something special about 時間 as a word that excuses it from counter, or are the rules inconsistent, or is there a pattern that I'm missing? I'm aware of 時間 as the word for hours, but as in Chocolate's answer, we don't say 一時間間.