You can typically drive on a P0420 indefinitely unless your state emissions control tests you, or the cat is so clogged that it produces enough restriction....note if it gets clogged enough you may burn a valve. How long you have depends on if you're burning oil or if you're running rich. It could simply be a leak in the exhaust not allowing the cat get up to its efficient temp, but that's not usually the case.
If you replace I highly recommend going with a direct bolt on magnaflow and replacing your O2 sensors.
The cat on my 2002 S went at 81,000 miles. I've been told this is typical for subaru cats. I guess consider yourself lucky it lasted till 120,000.