It would be six days at max, assuming they managed to steal the certificate immediately after it was issued, otherwise it’s gonna be even less.
Having the certificate doesn’t automatically mean you can change the site, if you have control of the site hosting you likely wouldn’t need to steal the cert anyway.
Stealing the certificate would allow you to run a man in the middle type attack but that’s inevitably going to be very limited in scope. The shorter time limit on the cert reduces that scope even further, which is great.
Since most Let’s Encrypt certs will have an automated renewal process this doesn’t even really change the overhead of setup so I think this move makes a lot of sense.
There are other things certificates can be used for as well of course but I’m just going off your example.
Why isn’t the lib politician tagged as lib? It’s so weird