I have been doing some digging as after 19 days I still haven't had a payout from my Smartnode yet. There seems to be very little information out there about how the payouts actually work (or I am not looking in the right places probably).
However, just now I found this post by @allemeister.
In it he says:
Note: There is a delay for first payment after the initial 10,000 smart collateral transaction. This delay is calculated by (2.6 minutes) * (number of enabled nodes).
Example: If I make my 10,000 smart collateral transactions to a new address for a fresh node with the current 8000 node count, it will take up to 14 days for the first reward.
By this calculation and considering there is now about 12,000 active nodes, I should get my first payment around day 21.
Here's hoping!