As long as it has two sets of antennae it is probably a Monarch. Queens have three. At this time of year your caterpillar is preparing to take a trip to Mexico, CA, TX, or FL, they are tanking up for the trip. Usually, it's 3 to 5 days as a caterpillar, 2 weeks as a larvae and 10 days as a pupae.
Herndon, VA (USA)
Take care of the small things....