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Use of present perfect with how long, for, since — reference

We use the present perfect in questions with how long and answers to these question. We answer with the words for or since.

"For" means a period, for example, for several days, for five years, for two months etc.
"Since" means starting from some date, for example, since Monday, since 1999, since school days etc.

Examples

— How long have you lived in Toronto?
— I've lived in Toronto for three years.

— How long have you known Isabel?
— I've known her since 2015.

BUT

— How long have you known Mike?
— We met in 2013.