When to change mower oil

I change mine at random.. Usually a couple mowings into spring.
if you are worried about acid buildup you should be changing even earlier?

As long as you keep oil in it and change at least every couple years you are doing better than most.
every two years here. I don't mow my lawn 25 times (hour per lawn) in the summer months. Besides, since I use mine mainly for trimming before the ride mower, it probably gets about a 20 minute usage each time I mow. The rider takes the rest of the hour, and done.
Depends on the season. This one has been hot, the grass isn’t growing, and I haven’t had to mow much. I probably won’t change it this year. With a normal season, or if I was also mowing someone else’s place to help, I’d probably do it. so this year gets a pass for the rider.

the generator has gotten use, it will get changed. I usually change everything in august/September.
I change mine about every 50 hours or two years, whichever comes first. Doesn't matter what time of year that happens to be but of course being mowers, it's not in winter. Never had a mower engine (wear) failure, it's always the deck or deck plus combination of other things. Both my mowers are over 25 years old.
I usually do it in the spring. There's no particular reason why-it's just that if I'm going to have to get the mower lifted up to pull the blade for its spring sharpening, I figure I might as well turn the thing all the way over and dump the oil out while I'm at it.

To be honest, I'd guess 90%+ of push mowers never have their oil changed(and B&S even advertises liftetime oil in their current engines as others have mentioned) so changing it at any point in the year puts you ahead of most of them out there.

I've "regressed" to 2-stroke Lawn Boys so don't have to worry about them at all now. With that said, I was getting ready to sell the Murray-branded B&S "never change" my wife bought new in 2018 and noticed that there were some pinholes of rust in the deck. Before I came along and started using it, I don't know what type of care it got, but I've certainly not abused it. It's also a $200 mower, but still the engine is still going strong while I'll go out on a limb and say that the rest of it is basically EOL.
'lifetime oil' ? WOW ! so no point in any oil changing because the engine will fail due to it's cheap construction/low quality before the 1st oil fill 'wears out' anyway?
'lifetime oil' ? WOW ! so no point in any oil changing because the engine will fail due to it's cheap construction/low quality before the 1st oil fill 'wears out' anyway?

Whatever the exact terminology they use, this is how B&S pitches at least their push mower sized engines. Just check and add oil, and they claim you never "need" to change it(even though I do). I think I've seen the specs that they consider "lifetime" 150 hours, which is probably 4-5 years for most people.

As I said, with the one I'm dealing with, there's nothing wrong with the engine but the deck it's on is rotting. That's probably not unusual that the engine outlives the deck, in which case they don't really have any motivation to make the engine last longer.

Most people aren't on BITOG, though. I'd not be surprised if I'm the only person on my block who's changed the oil in a push mower in the time I've lived here. My FIL has an MTD with a Honda engine that he use to have tuned up and have the oil changed annually, although I now do all of that for him(and he went a few years without it at all when the mower shop he used closed). A lot of mowers probably go their entire life without having a single oil change...

Of course as long as I can keep my Lawn Boys running, I'll hopefully never change the oil in a push mower again, but that's a bit of a different story.
The push mower in my signature was acquired from someone who never changed the break in oil. I suspect maybe 5 years/summers.
I serviced it every spring with Rotella T6, most recently T6 15w40.
Last year, I only ran it half the usual time and didn't change the oil this season... with BITOGs permission, of course (y).
All evidence (that isn't an oil analysis) indicates I can let it go this year and use it all summer next year before changing it.
So it is "at least two seasons" for me, now.
The "IDEAL" way is to change at the end of season, so fresh oil is in the mower while stored and then again before mowing in the spring.
I change it in the spring, some never change it and seem to do well, I think that as long as one uses top-notch oil any system is going to be OK.
So you change the oil at the end of the season, then let the mower sit all winter, then drain the oil out that you put in before the winter that never got used and change the oil again?
About every two years. Oil stays surprisingly clean for how much dust and bits are thrown around by the mower. Air filter gets pretty dirty and has to cleaned a lot.
I prefer to change the oil at the end of the season so that I don't have to think about it at the beginning of the next season. The mowers and rototiller get a change in the fall, the snowblower in the spring provided it got any use over the winter. As for the generator, I go by the hour meter since it doesn't have a season per se. The snowblower saw no use at all last winter but will likely be run for a warm up and then changed at least every three years. Same thing with the generator: if it doesn't accumulate 50 hours in three years I will just go ahead and change it.

Most of my OPE does not have hour meters, so I just make a judgement call on whether it should be changed at the end of the season. Pressure washer, leaf vacuum, trim mower, etc. I believe that my entire OPE fleet could enjoy an oil change using only six quarts per year, which is what my daily driver takes twice per year.
3 lages and all anecdotal experience. Has anyone ever sent 1 or 10 season old oil to blackstone to be analyzed to conclude our argument?
I change it after the first mow in the spring, just how I've always done it. I tip the mower to drain.
I put about 20-25 hours on my Toro, with a B&S engine, each year. When I first purchased it, I changed the oil every fall. After the first two oil changes, and noticing that the oil looked almost fresh, I backed off to changing the oil every 2-3 years.

When I do change the oil, I like doing it when I put it away in the fall, because it is just part of the process of getting it ready for storage.