I haven't tuned my piano in a while, even though the seller said it would have to be tuned once a year. It is at least 5 years old. How do I know when the piano needs tuning? I use it very often (almost every day), so I have gotten used to it and will not be able to tell if something is up.
Interesting question! You've got used to the sound, but you'd probably hear something amiss. Check each note (name) over all the octaves, check each note against its 4th and 5th, like a tuner does. Play C and G, play C and F. There's a sort of hard sound involved.
There is more likely to be a little bit of out-of-tuneness on notes with two and three strings. It sounds like a chorus effect, oscillating slightly between louder and quieter. If you hear that, especially on the upper part of the piano, it needs a tweak.
Worth looking at it every couple of years, so an expert can spot anything likely to need a look at - sticking keys, poor dampers, pedal adjustment to name a few.
If you need it at concert pitch, it's quite easy to check notes with a tuner or tuning fork.
Regular tuning is healthy for the strings. If they go too long without a tuning it may be difficult for the tuner to get it back to concert pitch. At the very least he may break a few strings.
I get mine tuned twice a year with the seasons. Although sometimes in the summer it can use it twice but I'm okay with it.
Think of a tuning like getting your car's oil changed. It is something you do every three thousand miles or so. Once a year is good.
I don't think you can tune them too often. There is a local theater near me and their piano is tuned every time it is used for concerts which is about four or six times a year. Artists who travel with their own pianos have them tuned for every concert. I think people such as Chick Corea travels with his own tuner.