Even if it's for an extended period, the carriers can be relieved one at time within the normal rotation cycle of the carriers.
I'm sure they can, but with 7 of 10 deployed, at some point you're going to run out of carriers to cycle through the rotation even if you extend deployments. Not to mention the current issues within the USN* regarding the sustainability of their Op Tempo.
Put it another way, if they could keep 7 out of 10 carriers at sea without any impact, why don't the USN do it all the time?
*Not that other navies don't have that problem either.