In the Meraki “Sponsored Guest” flow, the sponsor can only approve or deny requests the access duration is a global setting configured on the SSID splash page by a dashboard admin. There’s no built-in option for the sponsor to set a custom expiry per guest at approval time.
If you need flexible, per-guest durations, you’d have to move away from the native Meraki sponsored portal and use an external captive portal with RADIUS authentication. In that model, the portal can present the sponsor with a duration selector and then pass the chosen value back as a RADIUS Session-Timeout attribute (optionally with Change-of-Authorization support) to enforce it.
Alternatives inside Meraki without custom development are limited to:
Creating multiple SSIDs with different fixed expiry times, or Pre-provisioning guest accounts with different expiries for the sponsor to hand out.
Bottom line: native Meraki sponsored guest does not support sponsor-defined durations you’ll need either an external portal or a workaround with predefined durations.