do LCs and GCs just not break in a non-user repairable manner?
Depends on the brand, most major brands have good service and maintenance manuals for the flow path hardware of instruments, but if the electronics break it's usually a support call, so they can hunt down and replace printed circuit boards.
If you have several instruments, or poor local support, you are more likely to hold spares etc, and learn more complex maintenance.
Based on my experience, I would allow 5% of purchase cost per annum for maintenance for the first 4 years. that would cover parts you replace during maintenance, such as pump seals and valves, injector seals, DAD lamps, GC gas seals etc.
I would increase that to 7% for subsequent years for an HPLC, but keep it the same for a GC. If you are happy using non-OEM parts, you can probably halve the figures.
If your time is charged, then you can do a comparision with the serviceperson's rate, just multiply the time he takes by 2 - 4, according to how skilled you are.
It's hard to suggest values for electronic components, because they are much more senstive to the local electrical and laboratory environment. If the instruemnst are on good-quality conditioned power, and in a filtered Air Conditioned room, they have few problems.
If they are in a converted office opening onto the factory floor, you should allow for about 5% per annum for replacement boards and dust removal.
If instruments are deemed "critical" in your business risk assessment, you should have one of the following, instrument redundancy, a strong service service agreement, or a good relationship with the serviceperson.
Think of it this way. If you budget for the worst case scenario, you'll have money to spend at year end, but if you budget for the best case scenario, Murphy comes visiting, and you have a lot of grovelling and explaining to do.
Bruce Hamilton