We often use for and since with the present perfect to talk about things that have continued over a period of time up to present (or the recent past). Compare:
It has been raining for three days.
It has been raining since Monday.
We use for when we mention the lenght of the period (eg four days)
We use since when we mention the starting point of the period( eg Tuesday)
for + lenght of time
for nine days
for a short time
for two years
since+ starting point
since 3 o'clock
since July
since 1987
I have been here for six days
I have been here since 4 o'clock.
