DHCP server is configured to hand out addresses with a lease time of X. All platforms seem to receive a lease time that confirms to whatever has been set via the DNS / DHCP management console or iManager. Apple iOS devices, however, get a three (3) day lease no matter what the lease time is set to.
Working as Designed.
If lease times MUST be limited to a certain time, change the max-lease-time setting at either the subnet or pool level.
Although the DHCP server can be configured to hand out addresses of a certain lease duration by default; this doesn't stop a device from requesting a longer (or shorter) lease.
iOS devices, for example, request a three (3) day lease time when they make their initial DHCP request. Since the three day request is not greater than what has been set by the max-lease-time setting at the subnet or pool level, the DHCP server will honor the clients' request.