For は、数字を伴った期間を表す語句が来る（期間の長さ）
three hours
two days
a week
a long time
a few
several
many
Since は、動作や状態が始まった時点を表す語句が来る（地点・～からずっと）
three o'clock
yesterday afternoon
last week
last night
2000
then
* Sinceの特徴は接続詞として使われ、その後に主語＋動詞～が来ることもある
I have known the singer since I was a child
The man has lived in Nagoya since he came to Japan
