Nuvi pulls the data all day, every day, and as frequently as Facebook and Instagram permit. It technically comes in batches, usually, at least every minute. However, the more Facebook pages or hashtags you add to your watchlist and monitor(s), the more spaced out the queries for each one become.
Example: When optimizing your monitors for Facebook and Instagram, if you have 1 hashtag on your watchlist and monitor(s), Nuvi can query it at a high frequency (around once per minute). If you have 30 hashtags, they each get queried 1/30 of the time (around once every 30 minutes). That's simply the restrictions of the data supplier.