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

1. I haven’t seen my best friend **for** six months.
2. I’ve been at this school **since** 2011.
3. I’ve been in this room **for** half an hour.
4. I haven’t eaten **since** 7:30 this morning.
5. I’ve known my best friend **since** 2008.
6. I’ve lived in this town **for** ten years.

You can replace the details to personalize them further!