No one is going to tell you exactly the right time or moment to make that move and pay the fee to get in. It really is up to you.
I advise people to design an internship/apprenticeship phase for their career when they move to LA. A lot of commercials are non-union these days so there is an opportunity for you to go out and get audition practice while you’re not in the union.
But you need to join SAG-AFTRA if you want to work in TV and Film.
It’s up to you to decide when you feel ready to go comfortably and confidently in front of casting directors. You have to decide when you can show up on a set and do great work. And when you can show yourself as a professional who can hold their own with a cast already in motion…
That’s when you should join the union.
Conversely, a lot of actors wait and it holds them back. Being SAG eligible is another way of saying you are not in the union. Many producers won’t hire actors who are “must joins”. I tell actors to look at it this way: Writers are members of the WGA, Producers are members of the PGA, the main cast members are in SAG•AFTRA, the crew are members of IATSE. Why would a show, whose entire cast and crew are in a union hire someone who’s not?