http://www.monarchwatch.org/biology/cycle1.htmGenerally after they reach the 5th instar they will go off the plant and find a place to pupate (attach). There they will turn into a chrysalis. Depending upon how warm it is, they will emerge in 10-14 days.
There's move info on the above website.
If there was not enough milkweed for them to finish their cycle, they may pupate anyway, but the butterflies will be very small.