How long does it take you to acquire 20 Zetas?

I'm at 27 assigned zetas, started farming when they were introduced. I just started buying things other than zeta mats with ship currency. (I did finish off Maul and Vader right at the beginning) My ships were waaay behind the curve for a long time because I couldn't do the challenge for quite a while, but the store is guaranteed and drop rates are not