The crucial thing is knowing your user and the business process and then test, test,test and test,test,test in various environments. I use the silence timeout and the maximum duration as a failsafe, that is if the assistant doesn't hangup and the user doesnt realize. Then the maximum timeout kicks in. The key is to have it set to both minimize unnecessary minutes and also not to cut off the call early. Again the only way you can do that is iteration and knowing your business requirements