FOR + period expresses duration (HOW LONG?)
SINCE + starting point says when the action began (SINCE WHEN?)
 
1- I've known her for three days (for how long? for three days)
2- I've known her since last summer (since when? since last summer)

Compare:
- Now it's April, so I've been living here for three months
- Now it's April, so I've been living here since February

We can use FOR with any tense, but when we express duration from the past up to the present (past and present action) we use FOR with a present perfect, not a present tense. FOR with a present tense expresses duration from the present up to the future.

- Mike is here for two weeks (= He will be here for two more weeks starting today)
- We'll go to Rome for three days
- I've lived here for three years  (not: I live here for three years)
- She's been studying for five hours, tell her to have a break

In informal conversation we often leave out FOR

- I've been waiting for you (for) two hours!
- She has worked only (for) three hours today


SINCE is usually used with present perfect but it can also be used with any past tense depending on the situation. It expresses that the action began at that moment and continued up to the present (or the past time we are talking about):

- We have been married since 1998  (since 1998 until now)
- They bought the car last month. They had been comparing prices since Tom's birthday.  (since Tom's birthday until they bought the car)
- I've been waiting for you since 7 o'clock!   (since 7 until now)
- Josh worked there since last summer, but now he works here with us  (since last summer until he came to work with us)

For other situations (present or future) we prefer FROM

- The shops will be open from 8 o'clock in the morning
- I work from 8 to 4
- Our new company will start operating from September 10th

Note for Spanish speakers

En español, cuando expresamos duración, casi siempre suprimimos el "durante":
- I've lived here for two years =he vivido aquí dos años
En inglés coloquial el "for" también se puede suprimir, pero no es tan frecuente y no siempre se suprime (así que es más seguro no suprimirlo en inglés):
- she's been in Malaga six weeks (=for six weeks)
 
LLEVAR: En español, para expresar duración, es más frecuente usar "llevar (+ gerundio)" que "durante":
- It's been raining for two hours = Lleva dos horas lloviendo (= ha estado lloviendo durante dos horas)
- I've been waiting for you since 10 o'clock = Llevo esperándote desde las 10
- she had been waiting for you all day = Ella llevaba todo el día buscándote

Cuando se usa "durante" o "desde", el español a menudo usa el presente, pero en inglés suele ser el present perfect (pretérito perfecto):
- I've lived here for two years =Llevo dos años aquí
- I've lived here since 2006 =Estoy aquí desde el 2006