Monarchs

I was looking at all the milkweed plants coming and what did I see but 2 caterpillars. Where have they been cos i have worked in that flower bed. Which reminds me tomorrow I am planting the seed I saved from last year.  I need a whole bed full to feed them. They eat a lot and poop even more.  So they are in the house in their cage and happily eating.  How long til they cocoon?