Use for + how long: for ten minutes, for two years.
for + how long
They've been married for twenty years.
Try it: Woman: You're really good at tennis! Man: Thanks. I've played ___ ten years. Choose what the man says. (since / for / at) Answer: for — Ten years says how long, so use for.
since + when it started
Use since + when it started: since Monday, since 2020.
It's been cold since Sunday.
Try it: Man: Do you want to go to bed? Woman: Yes. I've been up ___ five o'clock. Choose what the woman says. (for / at / since) Answer: since — Five o'clock is when it started, so use since.
How long have you …?
Ask How long have you + past participle … ? about the time up to now.
How long have you known her?
Try it: Man: ___ have you been a nurse? Woman: For five years. Choose what the man says. (How long / How much / When) Answer: How long — She says how long, so he asks How long.
for or since?
How long takes for. When it started takes since.
for two weeks · since last Tuesday
Try it: Man: Sorry I'm late! Woman: That's okay. I've only been here ___ a few minutes. Choose what the woman says. (ago / for / since) Answer: for — A few minutes says how long, so use for.