So most of my experience is with cup and cone bearing, which are pretty easy to maintain. With newer, cartridge bearings I haven’t been paying much attention to it. Have just been riding until there’s a problem. Assuming that as long as there’s no issues, it’s fine.
Curious how often people are doing any kind of maintenance for the more modern, sealed/cartridge style bearings for wheel hubs, headsets and bottom brackets. And are you trying to clean and repack them with grease, or just replace as needed?
for me it depends on the bearing in question, and the type of bearing.
headsets - i try to pop the seals and fully repack before building up new bike and can service similarly as long as they are accessible (without cutting brake hoses) and not rusted out/too far gone.
bb - ceramic ones get serviced as per mfg instructions. something like a shimano or sram dub bb gets used until worn out and replaced.
hubs - replace when worn out, i do not service these. I will try to put and maintain fresh grease layer on outside of seals where possible though to ward off water ingress.
I generally try not to ride in the rain and I don’t power wash my bike so my sealed bearings seem to last virtually forever. The only ones I maintain is my Campagnolo bottom bracket - cleaned with fresh grease once every couple of years (these are serviceable ceramic bearings).
It would depend on the bearings.
Some are sealed tight, others are not quiet fully sealed. I would check with the manufacturer as to what they recommend.