YES..there is a "idle-timeout" for dsl customer's too on aggregation router(configured under virtual template interface) but it will not buy anything for you...
Only PPP will be re-negotiated once the idle-timer is expired..It will not disconnect the line..
so in case of DSL world (always on connection), there is no use of "idle-timeout" feature..unless you want different ppp parameters for users to be negotiated after idle-timeout..