It's almost summer. I just 'fixed' my AC by replacing one of the two time-delay 20 amp cartridge fuses in the box next to my condenser. The circuit breaker had not popped. The other fuse had continuity. No external signs of failure on the one did fail.
The AC has now run fine for 4 hours with the thermostat set at 55deg, is it a good bet that fuse simply aged out? Or should I investigate further?
You don't mention the size of the breaker for the condenser circuit. If the fuses are 20A and the breaker is 30A you'd obviously expect the fuses to blow first in an overload condition.
If the fuses are 20A and the breaker is also 20A you'd still expect the fuses to blow before the breaker in an overload condition; generally fuses blow faster than breakers of the same rating in an overload.
If the fuse is correct for the equipment and there was an overload that blew the fuse, it's possible your condenser is on the way out but it's also possible it was a one time glitch.
Years ago a lot of condensers required fused disconnects but you don't see it in recent years. Maybe your condenser is old, or maybe there are fuses there from an older unit that's been replace - if that's the case they may not be necessary at all (the breaker may be adequate without the supplemental fusing in the disconnect) or they may not be sized correctly, just left there from the old equipment.
I'd have replaced both fuses (you now have one new one and one old one) and if it blows again, have the condenser serviced / checked. I would not keep feeding it fuses indefinitely.
(Fuses may seem old fashioned but fuses are still very reliable protection. When they're old, maybe fuses may get a little faster to blow, but breakers can get a little slower, or may get a lot slower, or may not blow at all. Old fuses may cause a nuisance but old breakers can be dangerous.)
Fuses generally last a long time, though they can fail over time according to @ThreePhaseEel. They are basically "sized just right" pieces of metal. I am pretty sure many of the fuses in my house are the originals from 60 years ago. If anything, circuit breakers are more likely to fail over time as they are more complex (in the case of GFCI or AFCI, much more complex) devices.
I would be shocked (and be lucky that you were NOT shocked!) if there were external signs of damage to the fuse. Fuses normally fail totally on the inside. If they fail on the outside - scorch marks, cracking, etc. then that is a sign of a huge overload, which is not a good thing.
If the fuses are closer to the condenser than the circuit breaker (i.e., current flows from the meter through the circuit breaker to the fuses to the AC) then I would not be concerned at all. If it is the other way around (meter to fuses to circuit breaker (that didn't flip) to AC) then I would definitely get things checked out. Normally the protection closer to the device is designed to fail first and to fail more directly. For example, a circuit might have a 20 Amp fuse (or circuit breaker) and that circuit, together with several other circuits, may be protected by a 60 Amp fuse (or circuit breaker) - the idea being that each circuit has its safe limits, designed to protect both the circuit wiring and the attached equipment (like your AC) but that the circuits combined together have another much higher limit (perhaps based on service wire capacity).
Bottom line: If a fuse blows (or circuit breaker flips) just once then it is typically no big deal - perhaps an overload due to motor startup problems. But if it blows a second time then you really need to figure it out (in this case, probably getting professional service of the condenser).
