I service my non-cassette type bearings once a year. That may be overkill for some but that's my personal preference. Better to clean too often than replace a damaged hub or BB. I was on a tour a couple of years ago where we had to cross a stream about 3 feet deep. Water got in everything. Right after we got back I serviced my bike and a couple of others. No damage was done but the hubs and BB's had water in them and just leaving them alone would have eventually damaged them.