Kabby, Monarchs are usually caterpillars for two to four weeks. It kind of depends on the quality of the milkweed they eat and I think weather factors into it, as well. They tend to be one of the more reliable species as far as their chrysalis goes, with them usually taking about ten days to become a butterfly. However, this is also dependent on the weather and I recently had one that took three weeks. But that was pretty unusual.
