I try to gather a few milkweed plants from nursery sale bins to keep in pots for the migrating monarchs also. Depending on our winters, many times the plants survive and come back in the spring, or just go dormant for a time.
The larva take 10-14 days to get ready to pupate. While they are larva, they eat the milkweed plants. Then they make their pupa. When they come out, they are butterflies that need nectar plants that do not need to be milkweeds.