Hi,
To start with, there is no round-trip time as such for CAD and UCCX. UCCX will use round-trip only for connectivity between:
UCCX and Call Manager
UCCX and Finesse Agent/Supervisor Desktop
UCCX and Social miner
UCCX 1 and UCCX 2 (in case of HA)
There is no "maximum" value at which the application will typically timeout or stop functioning. Rather, as latency increases, product performance will become erratic and, in the view of the user, become unacceptable. For instance, agent state and call state will lag, the call buttons on the desktop will be slow to update with correct state. If using Cisco Supervisor Desktop, then report displays may lag, or the agents may disappear from the left side navigation.
You may be interested in looking at below section from the SRND based on which you decide the best type of WAN link you want to go for from the agent and UCCX connectivity perspective
Regards
Deepak