How often are you confused about using since and for in perfect tenses? For example, fill in the blank in the following sentence: He has been reading the Mahabharata _____ (since/for) 6 hours. The answer is for. If it's yours too, you may shout, 'Hurrah!' But, hold on, I have a question for you: Why can't we use since in the above blank?

Whenever such a question nudged me, I turned to grammar books. They laid out several rules regarding these two words.

Yet the uncertainty lingered on...

Then, one day, the Muse of Grammar pitied my miserable condition and blessed me with a simple formula that cleared my doubts forever and helped me get rid of all rules.

Here it is: 1. Use since when referring to the starting point. 2. Use for when referring to the amount of time. Consider the following sentences:
1. He has been reading the Mahabharata for 6 hours (the amount of time he has spent reading).
2. He has been reading the Mahabharata since 3.30 p.m. (he started reading at 3.30).

Does it help? Let me know. And do share if you have a formula of your own.