Fourteen days from the first charge.
You have fourteen calendar days from the date of your first successful charge on a subscription to ask for a refund. Monthly or annual — the window is the same. You don't have to give us a reason. If you write to us inside the window, we issue the refund.
The window runs from the first charge on a given plan, not from every renewal. If you renew an annual subscription a year in and decide the second year isn't for you, that is a cancellation rather than a refund: it takes effect at the end of the current period, and Essay II below explains what happens to your data when it does.