Here are the completed sentences with "for" or "since":

1. Have you lived in your flat **for** a long time?
2. We’ve owned our car **for** over twenty years!
3. She’s worked here **since** 2015.
4. We haven’t had a holiday **since** two years ago.
5. I haven’t missed a day at work **for** over a year.
6. I’ve studied Spanish **for** two years.
7. He’s been a baseball fan **since** he was a child.
8. I’ve had a cold **for** over a week.