How to add the time duration for scheldule call

How to add the time duration for scheldule call

Hi, 

When we are planning phone calls with leads or customers.
Depending on the client, lead and the reason for the call the time planned is always different from 15 minutes to 2 hours. Currently, the system does not allow us to change or add a time duration for a call planned. How do we do to add the aspect of time duration every time we schedule a call ?