How do I know when bearings need to be replaced?
Should I go by time? Or is there a symptom to guide me?
I refurbish a lot of older bikes for re-sale. The first thing I do is to pop off the wheels and manually turn the axles to see if the bearings are suspect. If the axle turns roughly, has side-to-side play, or is hard to turn...They probably do.
There should be "no more friction than supplied by the grease" when turned by hand. You cannot do this with the wheel on the bike. If you have the tools; cone wrenches and such, then a wheel rebuild is both easy and cheap. The best-grade #25 bearings are inexpensive. If the cones are shot it's a different story.
Time? My police patrol bike, with well over 3000 miles, still feels perfect. If you ride in the rain a lot, or ride off-road through mud and sand and through water crossings, naturally you're going to need more frequent maintenance. Bearings are something to check say...Yearly for most riders.
It depends on the kind of bearing, but I can offer a little hint.
The easy ones are the open bearing, the kind that comprises a race, a cone, bearing balls (or rollers) and (optional in many cases) a ball/roller cage or retainer. These are common in non sealed hubs, non sealed headsets and non sealed bottom bracket.
For these types of bearing, proper disassembly and inspection gives the best diagnosis. All components should be cleaned and degreased with a suitable solvent. Then close inspection is what follows. Do this in a rather clean work area, with good light. Look for cracked, bent, corroded, chipped or otherwise abnormally worn parts.
Depending on the materials and procedures used on fabrication, some items are prone to get some weird wear patterns. (For example, during a time, here in my country was difficult to get another kind of wheel bearing than a Chinese variety, where the balls quickly developed tiny pores, which in turn led to ball breakage. Another kind had balls that used to "peel off" like an onion).
Any rough surface in any of the parts justifies replacement. In short term or emergency, it is ok to replace only the damaged part, but chances are, the other components have received damage as well, even if it's non perceivable, so for the long term (or heavy use) the best is to replace all components.
This inspection can be part of scheduled or preventive maintenance, but should also be performed in case grinding noise is heard when spinning the suspected axle, or when "ticks" or "clicks" are felt from normal operation or external inspection. This means that every "now and then" you should spin your wheels while hand-holding them by the axle, steer the bike while lifting the front wheel (or with the bike in a workstand) and spin your cranks delicately by hand when the chain is off.
Sometimes, noise or rough feel is caused by sand and other debris that managed its way into the bearing and is stuck in the grease, so, cleaning and applying fresh grease is enough to solve the problem, provided the symptom wasn't left too long unattended. Cheap bearings and good ones with worn out seals are specially prone to get dirt caught inside.
These kinds of bearing should be properly adjusted in reassembly. The cone should press consistently against bearing balls, but not so tight as to cause seizure, and the retaining nut should be tightly torqued against the cone to prevent self tightening/loosening of the assembly. Take into account that when adjusting the retaining nut, the cone will get a bit tighter against the bearing, due to the existing slack in bolt-nut threads.
The most difficult (IMHO) type of bearing es the sealed one, as it usually is press-fit into its receptacle and almost for sure it need fairly specialized tools for removal/installation. However, this kind of bearing is usually much better made and from higher quality materials, so they last much longer and need less maintenance (if any).
The same symptoms reveal a faulty bearing: grinding, ticking or clicking noises, notched or stepped rotation, jerk, or loose feel. As the "sealed" usually means no maintenance can be performed, replacing the bearing is normally the best/only solution.
Sealed bearings are common in high end hubs, bottom bracket cartridges, integrated headsets and pivot points of some rear suspension designs.
The only way I can tell is to put the bike on the cycle stand and use a mechanics stethoscope to listen to the bearings as I roll them. You can also find bearings in need of clean and lube, but this requires a trained ear.
By placing the stethoscope end on various bearing mount points or housings, you can hear the condition of the bearing as you roll it. The faster you roll it the better.