In average half of the certs would expire in half of the time. A 3.5 days sustained DDoS attack would cause half of the sites using a 6 day certificate to be offline.
https://letsencrypt.org/docs/rate-limits/
For the “exact same set of hostnames” (aka. renewals) the rate limit is 5 certificates every 7 days.
So you could do it every other day, if you can make sure there's only one client doing it.
And they're very clear this is a global limit: creating multiple accounts doesn't subvert it.
So you'll need to manage this centrally, if you have multiple hosts sharing a hostname.