You can safely assume that the activated carbon is spent after one
week. After that it's simply a biological filter medium (and not a bad
one at that).

Yes, one week. No, not kidding.

Methylene blue will work to test the carbon, but is toxic to plants.
Just a thought.

Regenerating activated carbon is rarely if ever worth it. I'd suggest
running carbon once every few months, for one week at a time, then
discarding the carbon (it's good for compost, if you're in to that
kind of thing).