For — a period of time

Example: We have lived here for 10 years now.

Since — a point in time (in the past), until now.

Example: I have been working here since 7 o'clock.

1. It's been two years for since Kate and I became friends.
2. They've been together for since a very long time.
3. The famous street artist Banksy hasn't made any new paintings for since last winter.
4. I haven't been to an art gallery for since ages.
5. Harry and his sister have been making art for since their parents brought some crayons home.
6. I've been in this art camp for since two months.
7. Sam's been dreaming of becoming an artist for since he saw a TV show about Vincent Van Gogh.
8. The painting has been drying for since two days now.