For talks about the length of a period of time.
Since talks about the start of a period of time.
- I have been working here for three years.
(length of a period of time/three years) - I have been working here since 2008.
(start of a period of time/2008) - He was only gone for a few minutes.
- She has been working very hard since she started university.
(start of a period of time/start of her time at university)
