In your network, go to Wireless -> Splash Page and your SSID you are working with and at the bottom of the page you should see "splash behavior." From there you can change the splash frequency.
I believe the system tracks by MAC Address. You can go to the client and see when it was authorized and when it will expire.
Note that with iPhones I have seen an issue with the private network address feature where it creates a custom MAC address for the device. These typically cycle once a day and I have seen some of our users need to reauthenticate over and over until they turn that feature off.
Here is a link to the splash page options as well: https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Splash_Page
