'for' proper time ke liye aata h ?

I think, you're talking about the uses of "for and since" in perfect continuous form of tense. For - is used to show "amount of time" like, how many minutes/seconds/hours/years. Ex:- I have been studying for two hours. Since - is used to show "a particular point of time of starting an action/event", like, last Tuesday, 2009, midnight. Ex:- It has been raining heavily since last Monday.