APM SSO Multiple SP's utilizing single IDP - Inactivity Timeout

I am not sure that scenario is possible. SAML is the standard for authentication - but IDP is not a proxy of user application connections - it merely issues an assertion and sends the user on its merry way to access any given SP. When APM acts as an IDP, user has a session with APM, and that session has its own timeout. SAML as a protocol does not have any mechanism built in, as far as I know, to keep track of user activity and associated timeouts - which is why I am not seeing how the scenario you're seeking is possible to accomplish.