This is not at all normal, and chances are that something else is making it fail. They need to do more diagnostics. Causes could be: running rich due to another component failure, antifreeze leaking due to a head gasket or other failure, impact damage, excessive idling (such as, running for 1/2 hour or more without moving).
Is this recurring fault code P0420? Any history of O2 sensor replacement or engine repairs? How does it run? Any other fault codes or misfiring/stalling/valve noise issues?
Has the PCM been reflashed to the latest and greatest part #? Some Pacificas had a recall (F44) for fuel pump siphoning problems and a PCM software update.
I would gather your paperwork together, make copies, review and discuss with the dealer service manager the next step. This is unusual and like Bob, I feel that there is another issue causing the short converter life (30K) that may be missed.
These are covered by an 8/80 extended federal emissions warranty, although the final word would be up to the service manager, possibly depending on the root cause found, if any.
P0420-CATALYTIC 1/1 EFFICIENCY
When Monitored: Engine Run time greater than 90 seconds. Engine Coolant greater than
70°C (158°F) Vehicle speed greater than 20 MPH and less than 55 MPH. Engine Speed
greater than 1216 RPM and less than 1952 RPM.
Set Condition: As catalyst efficiency deteriorates, the switch rate of the downstream O2
sensor approaches that of the upstream O2 sensor. If at any point during the test the
switch ratio reaches a predetermined value a counter is incremented by one. One Trip
GOOD TRIP EQUAL TO ZERO
VISUALLY INSPECT CATALYTIC CONVERTER
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR