Migration is based on data size, but setting up the API server on a new VPS is quick... I'd say less than one hour from a fresh image on Digital Ocean. I'll try to publish a guide for that part.

Take a look at the full migration guide here: https://parse.com/docs/server/guide#migrating

It'll really depend on your app, and how many features you used at Parse. In most cases, it should be easier than rewriting your entire app with Firebase, but you should carefully look through the guide and make that decision yourself.