As noted by The Verge, Instagram has a new "Your Activity" section accessible by tapping the hamburger button on your profile page.
"Your Activity" displays the time that you've spent in the Instagram app over the course of the last week, providing a daily average.
There are options to set a daily time limit with a reminder to be sent when that time limit is reached, useful for those who want to cut down on their social media usage. This section also allows includes controls for managing notifications, with toggles for customizing the notifications that you receive.
Apple also natively monitors time spent in various apps like Instagram through the Screen Time feature that was introduced in iOS 12. Screen Time info can be accessed in the Settings app on iPhones and iPads.
Facebook, like Instagram, is supposed to be gaining an activity monitoring feature, but it hasn't yet been introduced.