I found a 2x 8 foot slimline lamp ceiling light in a house that was gonna be demolished.

Changed out over 180 5-10+ year old Sylvania F32T8 4100K lamps for 3500K T8.  Replaced 8 bad Advance 4 tube ballasts as well.  (about 20-25 years old)  Most were already changed out previously, these lasted this long which is surprising.  Used new OSRAM 4 lamp T8 ballasts.  Looks beautiful with the 3500K in this particular chapel. 

How long do magnetic ballasts usually last?

Can not be answered right away. Magnetic components have no obvious failure modes and can last 100+ years. Very much depends on how many 'cost optimizations' were applied . Modern cheap ones with winding temperatures reaching above 100C may burn out in less than a year. Also today's dry film polypropylene capacitors ultimately lose their capacitance over time because of the phenomenon called by buzzwords 'partial discharge' and by aluminum oxidation.

Daily use for 8+ hours a day, I've seen them last 20-30 years.  And I've seen them last less than 10. 

What I have observed so far, magnetic components really very rarely fail on their own. But practically always they stop working because they were literarly murdered. Most often by humidity and consequent corrosion (the corroding iron expands, causing winding strain, damaging both wire, but as well the magnetic steel plate surface insulation and consequent overheating), by wiring not be properly secured to tearing the connectors apart, very often by the applicationtorturing them by inadequate cooling (I'm not talking about operating a modern 150degC rated ballast at 100degC, but a 175degC rated one at 200degC and above once the fixture gets closed, because some moron in the fixture design was satisfied with a fixture design where the ballast was reaching 174degC with the cover still open).

Capacitors are, however, way different story. There the elimination of the PCB oils did cause the wear out to be the dominant factor, there is just nothing to really completely replace their chemical stability (well, that also is what makes them so problematic in their disposal).
Either by the PCB-free oils degrading, or later the dry film not offering the recovery characteristics the high temperature stable oil dielectric does.

Practical examples of magnetic ballasts dying prematurely without much abuse:

Cheapo 600mm 4x18W modular ceiling luminaires at my old workplace. Of course stuffed with smallest and the shittiest possible Chinese ballasts. EEI=B2 and this probably is still a fake index. Not a thermally challenging luminaire, not much power and quite large internal volume. At lamp EOL and stuck starters many of them got yellowed and become dead short and burned out. In my youth I have seen many old-school magnetic preheat ballasts, none of them suffered when they encountered lamp EOL.

Also, for some reason, a couple of 'sausage' 11W PL-L ballasts of mine developed short windings and excessive lamp current after may be ~ten years of use. Again not thermally challenged, first one of them might get hit by a lightning spike on a rural power network, though.

Regarding capacitors, back in the Soviet time paper / metal film capacitors were in use. They were made in hermetically soldered metal cases with glass insulators and could be regarded as very reliable. The filler was a simple benign hydrocarbon wax, based on the smell. They were large, for sure. PCB capacitors appeared briefly probably in mid-1970s and I remember that chemical naphtalene-like aromatic smell. This oil was called 'sovol'. Sovol capacitors were may be 2-3 times smaller by volume, probably as a result of high dielectric constant of PCB oil. At that time we did not know the stuff is so nasty.  Still, not very much risk from a brief contact with guts of a single capacitor if you don't eat them

Ballasts for remote-geared lanterns here in the UK have been known to last 50+ years..!

Replaced about 20 F32 T8s Sylvania 2014 vintage 3000K Warm White replaced with 2018 vintage new Sylvania F32T8 3500K 28W.  They look goooooooood.   Got hundreds more of these I'll be relamping in the coming months Only problem at this building whomever put in the ballasts, did not wire them up correctly. Programmed start but wired them as instant start. 

Looked at the Lighting gallery, then looked at the Oregon fluorescent lamp ban laws. Turns out they're more lenient than I thought.

Visited 8 restores in one day and got some good stuff. Also planned out another lighting thing that should happen this thursday, i shall not say what it is.