Ответы
1. I have been working here for half a year.
2. I haven't taken a pill since last month.
3. We were abroad since December.
4. He was travelling for eight months.
5. My dog has not been sick since winter.
6. It was snowing for many hours this morning.
7. It's been raining since Saturday after we arrived.
8. I was thinking about this for a week.
9. We have been using your help for last two weeks.
10. Since third century we lived here.
11. We were building this place for last three months of the year.
12. My friend have been explaining it for 10 minutes since you arrived.
13. He will work hard for three weeks after that moment.
14. We've been doing this for last three centuries.
15. Many things have changed since last month.
16. They were working since 3:32 AM.
17. I'm sorry for Richard, he was working on this project for days.
18. Since when did you learn Croatian?
Когда какая-то отметка во времени (непродолжительная) - чаще всего since.
Когда показан длительный отрезок времени - for.