To the best of my knowledge, EXOS doesn't support to manually set 'interim-update' interval. I think it can be determined by adding 'Acct-Interim-Interval' attribute in 'Access-Accept' message on the radius server side.
Unfortunately, it's confirmed that EXOS doesn’t support Interim update accounting message even if it receives Access-Accept with Acct-Interim-Interval Radius Attribute.
If you don't see an interim update with the 'Acct-Interm-Interval' attribute properly piggybacked in an 'Access-Accept' message to a switch, please contact our GTAC for further assistance.