"Dwight Sipler" wrote

It takes more than 10 days to kill the sod. I put black plastic down
over grass for a temporary greenhouse once and it was spongy to walk on
for about a month and a half.

Hmm. By my experience 10 days is plenty, but maybe it differs with climate,
type of grass etc. At any rate it's always an option to leave the plastic
down longer, but that makes it harder to water the new plants. Cheers.