How long does it take you to acquire 20 Zetas?

I think best way is buy zeta from ship shop. If you have maxed your mained fleet, so better farm Zetas. For example I dont need gauntlet, or rex arc, so I don't farm. So every day couse I finish in1 place I can buy 1 zeta. Maybe I dont do that but need soap aerated 2 heroes. From challenge zeta is like as... 1time I even from 3 weeks dont get even 1 zeta