PRESENT PERFECT SIMPLE AND CONTINUOUS

Present Perfect Simple	Present Perfect Continuous
Result (what / how much / how often) I have written 5 letters. / I have been to London twice.	Duration (how long) I have been writing for an hour.
Emphasis on completion I have done my homework. (Meaning: My homework is completed now.)	Emphasis on duration I have been doing my homework. (Meaning: That's how I have spent my time. It does not matter whether the homework is completed now.)
§  how often §  ... times	§  how long §  since §  for