5.4 Configuring Dial on Demand Routing (DDR)
5.4.5 Step 4 - Defining optional call parameters

The following additional call parameters can be added to the interface:

Router(config-if)#dialer idle-timeout seconds
Router(config-if)#dialer fast-idle seconds
Router(config-if)#dialer load-threshold load [outbound | inbound | either]

Idle-Timeout versus Fast Idle
Combining the
dialer idle-timeout and dialer fast-idle commands allows you to configure lines to stay up for a longer period of time when there is no contention, but they can also be reused more quickly when there are not enough lines for the current demand.

Also, if you intend to nail the line (make it a permanent connection), set a high timeout value on the receiving router because these commands apply to inbound and outbound calls.