For a post-1L transfer, ask toward the end of the second semester. You don't want to seem like you're being presumptuous by asking too early, but if you ask during finals it makes things a little weird. If you have good relationships with profs from fall semester and won't have them in the spring, keep in contact with them. Go to office hours and ask them for advice on your career goals. This serves the dual purpose of continuing to build the relationship so they don't forget about you when you ask for a rec, and giving them a little bit of insight into who you are as a person and future lawyer (not just a law student).
Asking too early also opens you up to potential sabotage attempts by the school. Nobody is ever really sure if this happens, but the dean of my old school sent a snotty letter to my new school in the same envelope as my letter of good standing toward the end of my transfer process. Keep it on the hush hush until it's application time (and even then, really).