Bearings maintenance at regular intervals or as needed?
 

I would think that bearings wear out gradually and that catastrophic failure is exceptional.

What kind of distance / time can we expect from headset, bottom bracket and wheel bearings (in my case Cane creek 40 and XT components). How much more can be expected from the early signs of excessive wear before replacement becomes imperative?

Asking in anticipation of rather lengthy tours.

Absolutely no way to tell - depends on weather and road (or no-road) conditions more than miles or time. I did a 10k tour in '76, overhauled one hub that was getting a bit dirty, replaced one chain and a shift cable. Generally one can tell by feel if a bearing may be needing attention. Outright replacement would require 10's of thousands of miles in some cases, much less in others.

Start by checking the freeplay and ease of turning on each bearing. Bearings should turn smoothly without play.

Remove wheels from the bike to check axle rotation and freeplay with your fingertips. Remove the chain to do a similar check of your BB.

For the headset, raise the front wheel and see if the steering freely falls to either side by its own weight. Also, with the front wheel on the ground, check bearing play by firmly engaging the front brake and pushing forward on the bike. You'll feel movement if the steering has any play. Remove the front wheel and rotate the fork in the frame, checking for notchy or slow movement. A defect will often show itself as "indexed" steering rather than smooth, uninterrupted turning.

With all bearings, feel for notchy turning, rumbling, or binding.

Service intervals are dictated by the type of riding you do and the effects of weather and road conditions on your bike. If I were going on long tours, I'd service any bearing that was doubtful.

I have a set of wheels with Atom hubs that I ride regularly. I have not done any bearing maintenance on these for over 20 years. The deal is that the axles still turn as smooth as silk. Every time I think about tearing them down, I do a spin with my fingers and change my mind.

I used to be obsessive about maintenance (meaning: take things apart, clean, regrease, and reassemble). Now my maintenance regimen is to frequently inspect, and do more only whenever I have a feeling that it needs it.

Agreed. When I suggested servicing a bearing that's "doubtful," I meant one that failed the tests I outlined.

I repacked the cup-and-cone BB on one of my bikes when I acquired it in 1980. This year, I decided to replace the Spindle because I wanted to go from cottered to cotterless cranks. I found that the 35-year-old grease was still okay, and the bearing had no play at all. However, this bike has seen little or no rain and had only traveled about 2,000 miles in 35 years.

I had one bike that ate a cartridge bottom bracket in less than a year. That taught me about the importance of drain holes.

Clean and repack according to conditions. A lot of wet and sand, do 'em annually or more. Sunny days only, every five years or when they complain.

The trick is, if you wait to long the bearing race will be damaged and will need replaced. Too soon and you waste a dab of grease and some time.

chipping pot holes out of the bearing races on classic loose ball hubs and BB is meaning you have to look harder all the time for replacements,

as they get Rarer and rarer.. so preemptive cleaning will extend the inevitable off a ways..