Setting a timeout is currently only possible for exec sessions, but not for PPP sessions. To set this timeout, "aaa authorization exec TACACS+" must be configured. The desired timeout can be set by the "timeout" (IOS >= 11.0) and "idletime" (IOS >= 11.1) attributes in the daemon for exec authorization. This means that exec authorization has to be enabled for these timeouts to take effect.
This timeout also has an effect on PPP sessions started on the exec prompt because the PPP session is also terminated if the underlying exec session is.
Alas, this is not possible for PPP sessions on dedicated lines, autostarted PPP sessions and ISDN PPP sessions. Up to now, it is not possible to set a timeout on them using TACACS+.