The only timer that you can adjust is the return to queue timer in the call presentation class assigned to an agent.
Take the scenario where a caller has been waiting in the queue listening to music and announcements. When the call is presented to an agent i.e. it is ringing the agents phone, then the caller hears ringtone. If the agent does not answer, then the call will be re-presented to the queue after the timer expires. If there happens to be another free agent then the call will ring that phone. If the return to queue timer is set for 20 seconds (which is a bit too long in my opinion), and the second agent does not answer for 15 seconds, then the caller has been listening to ring tone for 35 seconds.
An agent can of course choose to reject the call and return it to the queue rather than answer it, which further complicates things.
Hope all this is helping, I'm sometimes not to good at explaining.