In my experience if the exhaust is aloud to get hot by at least one decent run a month the cats should last many years.
Last year I had to replace the cat on a friends Ford to get it through an MOT, this car is used for a weekly shopping run, weekend family activity and once a month it gets a 200 mile run so his wife can visit her parents. The car has been owned since it was a year old, it has covered 140k miles and is 15 years old.
The cat needs to get up to full working temp to burn off all the rubbish and this can 5 mins so that quick run to the shops or if you only work 10 mins from home it is unlikely to last long if that is its main use. The other problem can be the case the catalyst is enclosed in, luckily on our cars this seems to be stainless steel but this will also degrade with the hot cold cycling especially if it gets splashed with or submerged in water.
The other thing to avoid is any impact on the exhaust system, this can shatter the cat, if you can here a rattle this is often the problem.
The only way to know if it is near the end of its life is to see how it does at the MOT, if the tester has to get it hot by revving it it will well past its best. If it passes on idle with no more than a 5 min run before the test it has plenty of life in it
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