How long does it take a caterpillar to turn into a monarch butterfly?

Actually, we have monarch caterpillars on our milkweed outside. It took only a few days, maybe a little more than a week of eating before they were fat enough and ready. At the moment, a few minutes ago, one of ours just started its cocoon.

yep P.S mine is starting its caccoon too! i got 2 but one lazy one hasent ate was much but hes slowly climbing up for to start at this momment so like hours ao my catapiller( it is a monarch) started... its make a little lot more of the stuff for it...its preparing its body for the cocoon now!