1 month
What makes you think a court can’t order them to modify the site/client to capture your key and send it home?
There’s already precedent for this. The FBI leaned on Lavabit to serve compromised code to Edward Snowden. Lavabit closed up shop instead.
Can read more here, which makes the case that “web-based cryptography is always snake oil”: https://www.devever.net/~hl/webcrypto
The cops have investigated themselves and concluded that they did nothing wrong.