Let's be real, the main usage of SG nowadays is to keep your Material Storage active and that's 300 SG per month. So let's see if we can figure how many per month we can get with minimal effort and no premium:
-60 per month just by logging on to PSO2 every day
-30/31 per month just by logging on to PSO2es every day
-44 per month with Arks Day
-Between 30 and 150 per month by using Casino passes and converting your coins into 10 SG every week
Unless I missed some, you can get between 164/165 to 284/285 per month with those criteria, add 60 more if you're premium.