We use the present perfect tense when we want to talk about how long something has continued. The thing must have started in the past and still be true now.

We use it to ask questions with 'how long'.

• How long have you worked here?
• How long has she been in London?
• How long have they studied Spanish?

When we answer the question, or make a positive sentence, we need to use 'since' or 'for'. 

We use 'since' with a point in time in the past: 2014, April, Monday, last year. 

• I've known Sam since 1992.
• She's been here since 2pm.
• We haven't seen Amanda since last year.

We use 'for' with a period of time: 2 hours, three years, six months.

• I've known Julie for ten years.
• I've been hungry for hours.
• She's had a cold for a week.