I use you are using ArubaOS (controller)?
You can try to extend the User Idle Timeout. This idle timeout will control the timeout of entries in the user-table. You can configure the User Idle Timeout with a maximum value of 4 hours. This means the client can remain disconnected for up to 4 hours before it needs to re-authenticate.
You can configure this globally: Configuration > SECURITY > Authentication > Advanced > User Idle Timeout.
Or you could also configure this on the AAA profile (added in ArubaOS 6.3).
Please beware: the User Idle Timeout can *never* be lower than the DHCP lease time or else this might cause issues with the "prohibit IP spoofing"-feature, which is enabled by default.
As mentioned by cappalli an alternative for this is MAC-caching by using a RADIUS server (like ClearPass).