How to calculate total supply after one week of lending on Compound Finance?

I ran a quick calculation that seems wrong. I assumed the money lent compounds every 15 seconds, and that led to 2 Ether growing to 2.56 Ether in a week. Is how often your money compounds dependent on the total supply? It sounds like an action needs to touch the funds lent to trigger a compound period.

On their website they just have some complicated calculators that are kind of hard to decipher down to the weekly total supply, after compounding.

I used A = P(1+r/n)^(nt). P = 2 Ether, r = 0.25 n= 40,320 (assumes compounded every 15 seconds), and t = 1 week.

How much should 2 Ether increase to with 0.25% APY? Would the math giving me 0.56 more Ether in a week be accurate if you put millions of dollars in?