Constantly wiggling moth pupa - will it emerge soon?
I posted the same question on tumblr and got an answer:


  It depends on the species. This one looks like a Noctuid. I’d give it
  two weeks to a month or so. You may be able to see its wings showing
  through the darkening pupal case when the time draws near! Just make
  sure you give it somewhere to climb up and expand its wings when it
  ecloses.


After keeping it until the moth emerged, I now know that wiggliness is not an indication of maturity, but turning dark is.