The time for when each individual may consider SAG-AFTRA membership is different.
As someone just getting started in the industry, it may not be something you want to jump into right away because it limits your ability to build credits. Once you obtain SAG-AFTRA membership, you must now comply with the union rules. Most significant are the Global Rule One and the "No Contract/No Work" rule. Essentially, once you become a SAG-AFTRA member, you cannot work nonunion gigs anymore. No more nonunion movies, TV shows, commercials, Internet projects, or student films. It is a global rule, no matter what country or where you are on the globe, you generally are not allowed to accept a nonunion gig.
In Hollywood, it is difficult to get an agent. It is even more difficult for talent to get booked on highly-competitive union jobs. The industry is tough. The industry is driven by credits and how successful was the last project you worked on.
To build credits, nonunion jobs may be the best option. But once you join the union, the member does not have this luxury to build its credits. Even SAG-AFTRA advises: "The time to join is when it makes sense for you. (Everyone's) situation is different." Each person considering union membership should get feedback from mentors, managers, teachers, friends, mentors, and coaches. #SAGAFTRA