Same day the subscription is renewed/bought, or at least it shows up eventually that day. EDIT: oh yeah, misread that, premiums do not get ANY SC at all, only legendary members.
Not sure if I used the right term. I paid 14.xx for the monthly membership with all the DLC, etc unlocked. I'm pretty sure I was also supposed to get 500 SC. It's been 3 days since I did that. No SC yet.
If you are on a US PC account, you have to manually claim your Station Cash each month from the Marketplace. There is a Button on the Top Right of the Marketplace screen that includes a Date. If you are on a US PS account, Loyalty points are automatically added each month.