Present perfect tense relates to an action that started sometime in the past and is still occurring in the present: As examples for 'how long'. 'for' and 'since', you could use: How long have you known your friend? I have known her for three years. (for = a length of time) I have known her since 2010. (since = a period in time) How long have you lived here? I have lived here for six years. I have lived here since January 2007. How long have you been studying English? I have been studying English for one year. I have been studying English since November last year.

"How long" and 'for' refer to some duration of time - 'how long' is more of a question form, while 'for' is more a statement. "How long have you been employed here?" "I've been here for three years." 'Since' is used to specify something that has occurred starting at a particular point in time. The duration is implied. "I have been employed here since 2010." If you did the arithmetic, you would find that that person has been working for about three years. Note that the time we are talking about may not be exact. "I have been working since August." I could mean any time in August - the 1st to the 31st.

for how long= ne zamandır since= o zamandan beri ( point of time. for example= since 1453) for= şu kadar zamandır. ( duration of time. for example= for 2 years.)