The developers hard-code a maximum 500 minute inactivity timeout, so you
don't have idle devices taking up your DHCP leases. Martin, if your firmware
only allows 1-10 minutes, you might be on 9.7.1/9.8.0 which had an ER-1553
temporary 10 minute max, reverted to 500 minute max in 9.8.1+. and like it had
been before 9.8.1.