I would say hours
The big kit would be fine for several hours with Dad at 18 days old.
My very first litter I accidentally put Dad with the kits and put Mom is dads cage. It wasn't until the next morning I noticed Dads dropping were some big, and instantly thought "they look as big as Teasers today
In the same thought I said no I couldn't have
But I did. the kits were only a couple of days old. When I looked in at Dad and the kits, he had them in the house all curled up with him. And I'm pretty sure I saw one of them eating out of Dads mouth.
I switch Mom & Dad back and they latched right on and went back into their house like nothing to it.
Now I don't reccommend you do it for over night, but a couple of hours maybe even twice a day shouldn't be a problem.