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.
To some extent, it depends on what you can put up with personally. But even if you're OK with how it sounds, there are still a couple reasons why you'd spend money on tuning:
You might find you enjoy the sound of the piano more once it's been tuned. A freshly tuned piano will sound clearer and more consistent from key to key. Given that you play it daily, perhaps this better experience is worth it? One thing you might do is record yourself playing a favorite piece before and after tuning. This will give you a really accurate idea of whether or not the sound of a freshly tuned piano matters to you.
Over multiple years, pianos are going to drop in pitch overall because the wooden parts gradually shrink over time. Unless the strings are rusty, your tuner should be able to pull it up to pitch without anything breaking, BUT you might have to pay more for it (it's more work). Plus the bigger the change the tuner has to make, the more quickly the piano will go back out of tune*. So assuming you did the test in #1 and decided you like the sound after tuning, that "freshly tuned" sound will last longer the more often you have the piano tuned.
* The main reason for this is that piano wire forms bends around each of the bearing points. When you change the tension in the wire a lot, those bends end up moving a little -- the old bend will relax a bit and a new one will form, which throws off the pitch.
Once per year is really a pretty good minimum tuning interval, because seasonal changes are more cyclical -- Let's take the example of a piano tuned in the winter: when the air conditioning starts to click on in the summer, it will increase relative humidity, which expands wood parts and makes the piano go sharp. Then as winter rolls around and you start using the heater, relative humidity goes down and the piano goes back to roughly where it was before, probably a little lower because again, wood shrinks overall across multiple years. By tuning it once per year you avoid this steady drop in pitch over the years.