When do I use 'since' and when do I use 'for' with present perfect?

Use 'for' with a period or length of time, and 'since' with a point or starting time. 'Five years' is a length of time, so write 'I have lived here for five years'. '2019' is a point in time, so write 'I have known her since 2019'. More examples: for two hours, for a week, for a long time; since Monday, since morning, since I was a child. A quick test: if you can answer 'how long?' with the phrase, use 'for'; if it tells you 'when it started', use 'since'. Both are commonly used with the present perfect tense, which links a past starting point to the present moment.