what is the serviceable lifetime of the following, and when should they be replaced?

I know this will sound like heresy to someone who is maintaining a car "by the book", but I have adopted a maintenance practice that has dramatically reduced the cost of keeping my cars running.

In essence, it is this: Drive it 'til it breaks, and then fix only what's broken.

Don't go looking for trouble. If the lambda sensor is still working, then why replace it before it stops working?

Obviously this is not a completely black and white philosophy. If your ball joints are loose, you don't wait until a wheel falls off. If your injectors are worn to the point of too rich a mixture, replace them. Consider partial failure to the point of danger or improper running to fall into the "'til it breaks" category.

However, don't get sucked into the "...as long as I have it apart I might just as well fix..." idea. Last night I'm pretty sure I blew a head gasket on my faithful old 1969 Toyota Land cruiser -- my snowplow truck. The cooling system pressurized to the point of first filling, then melting the overflow bottle, which in this case was just an old anti-freeze jug. ('69 Land Cruisers were before the days of sensible environmental practices, and the original system consisted of an overflow hose that just vented to the ground) I blew through three gallons of antifreeze in an hour and a half. So, I'll replace the head gasket, and will lay a straight-edge along the sealing surface and if necessary I will have it machined to keep from immediately blowing another gasket. But I will not do a valve job just because the head is off and it is easy to do. I will not take a ridge reamer to the cylinder bores. I will not use this as an excuse to pull the oil pan and put new main and rod bearings in it or pull the pistons out and replace the rings. Yes, all of that stuff is worn (c'mon, I've been plowing snow with that truck for more than 35 years, after all) but that stuff is still functioning. Yes, it would run better with new rings and bearings and valves in it -- but it runs well enough now to satisfy me.

When I applied this maintenance philosophy to my bicycles, I cut my operating expenses by just about an order of magnitude. And if you scoff at the idea of maintenance "expense" on a bicycle, you have never owned a full front and rear suspension bike with 27 gears, hydraulic disc brakes, and indexed shifters!

tanstaafl.