All bearings, independent of their type, should always be checked for each diagnosis (testing backlash, listening to foreign metallic noise). It may be noted that the replacement wheel bearing is a routine work, but it must be remembered, and the recommendations of the manufacturers. If you have any accompanying noise when driving or riding (difficulty of the wheels) – the first sign of "disease" bearings. But as soon as it happens depends on the quality of bearings in the first place on your luck. Plays an important role and coverage of roads (as far as bearings for cars) – is not always possible to go on perfectly smooth surface, hence the wear bearings. You ask the question: "How many bearings can withstand?". The answer – a lot and very little.
Year, and maybe even a season of active driving on the road will lead to bearing wear (whatever they are good build nebyli). From three to five years will serve bearings for the "walk and not training." Bearings can simply "kill" skating on the wet asphalt in the rain or after it. There have been cases that on the morning of the wheel roller is very difficult to turn and even a very strong your friend is not able to cope with it. In most cases you should not really worry about and experience – all fixable. I also want to note that the negative effect on the bearings have a large (Unintended) load – kicks, jumps. On personal experience I can say that timely maintenance, inspection in the diagnosis, moderate off-road riding – one hundred percent will increase the life of your bearings. Now there are many bearing producers, both domestic production and foreign. Impossible to give specific recommendations as to specific design is necessary to select certain bearings.