How often are hubs changed?

Why are you replacing the bearing assembly? Typically when they fail, they get noisy and make a buzzing sound similar to an airplane engine. The biggest reason for repeat failures is improper tightening of the large axle nut. You must use a click-type torque wrench on that nut and there must never be any vehicle weight placed on the wheel unless that nut is tight. Placing weight on that bearing when the nut isn't tightened to specs will make the new bearing noisy right away. Not having that nut tight enough will make the bearing noisy in anywhere from a few days to a few months. Most of those axle nuts call for at least 150 foot pounds. Some require up to 240 foot pounds.

@"Caradiodoc" but 'am told the bearing failed due to water, dust et al cos I think that nut was well tightened.

What was that failure? What was the symptom or problem with the original bearing and with the replacement?

Wheel bearings live in an environment full of rain water and dust from dirt roads, and they're built to survive those conditions. There is a rubber seal behind the bearing that keeps out water and dirt. It's common to install a new seal with a new bearing but if that isn't done, you just have to be sure it's in the proper location to do its job effectively.

The biggest problem is the quality of the parts being sold today at the discount parts stores. They are all stocking cheap, Chinese made junk. You have to buy a quality name brand like Timkin.