How long is synthetic oil good for?

If the car is used for a bunch of short trips and doesn’t get fully warmed up, that would decrease useful oil life, but if it’s used for a few longer trips, the oil would be good for a long time.

Another consideration I’d add is the age and potential future of the car. If you use a long (multi-year) change interval, that may reduce the chance that the car will get to 400,000 miles (or some other high number). With the car’s current lifestyle, it’s unlikely to accumulate a lot of miles. You might consider a shorter (one year) interval if you foresee using the car differently such that you might want hundreds of thousands of miles from it in the future. If you foresee only low-mileage use, long life (in terms of miles) is not a concern and a long interval may be suitable.