Using the present perfect, we can define a period of time before now by considering its duration, with for + a period of time, or by considering its starting point, with since + a point in time. FOR and SINCE can also both be used with the past perfect. SINCE can only be used with perfect tenses. FOR can also be used with the simple past.
For + a period of time
for six years, for a week, for a month, for hours, for two hours
I have worked here for five years.
Since + a point in time
since this morning, since last week, since yesterday
since I was a child, since Wednesday, since 2 o'clock
I have worked here since 1990.
Present perfect with FOR
She has lived here for twenty years.
We have taught at this school for a long time.
Alice has been married forthree months.
They have been at the hotel for a week.
Present perfect with SINCE
She has lived here since 1980.
We have taught at this school since 1965.
Alice has been married since March 2nd.
They have been at the hotel since last Tuesday.