Our upright piano has been last tuned about a year ago. (It was a major retuning, two sessions, including bringing it up from something 435-ish to 442 Hz.)
I just tried to check whether it is still in tune and I couldn't find any flaw.
It just sounds good when practicing.
Using tuning software (basic Android phone, Intonia Pro in tuning mode) I was able to check that every note (every struck key) is chromatically consistent with a 442 Hz tuning.
For additional science, I also checked that everything between A1 and C7 was visually detected as higher than consistent with 441 Hz tuning and lower than 443 Hz; there's a gradual 440-ish tail in the deepest basses as expected. I'm not sure I really checked the top few notes as carefully as the rest, none of these was off to the tuner or to my own ear.
Individually picked strings for each key were in unison, judging just by ear. No vibrato, no pitch differences. In this step I ignored the bass notes that have just a single string.
Apparently the professional tuner has done a very lasting job a year ago - or I'm missing something I should also check.
I'm tempted to consider this condition of the piano more than sufficient for amateur and student use and re-check its tuning only in another year or two.
Except that every online resource I can find advises to invite a tuner at least once a year, if not twice. Are such guidelines written simply to create more business for tuners? Or is this a marvelous instrument that's resistant to humidity fluctuations?
[After asking this question and learning things from the initial answers, I started to pay attention to ambient humidity. It seems to be fluctuating only between 50% and 30% over many months - from autumn to mid-winter. There's no fireplace, no AC. This low variance of humidity is apparently contributing to year-to-year stability of tuning.]