I'm talking about either beats per second of a resulting beat between two strings in the midrange of the piano. My piano seems to vary during different parts of the day, I think because of the temperature variation in the room where it sits.
You take a tuning fork tuned to 440Hz and a second one tuned at 442Hz, the beating will be 2beats/second and the following is true:
440 + 442 = 2beats/sec 440 + 437 = 3beats/sec 880 + 882 = 2beats/sec 220 + 219 = 1beat/sec etc.
So the difference in the frequencies equal the beating per second.
The beating itself is heard as a fluctuation of the volume of the sound (not the pitch)
When two strings a perfectly in tune the attack part of the note will sound harder or tighter. The harmonics (partials) also will be more noticeable thus augmenting the "clarity" or "brightness" of the sound.
As soon as one of the strings are getting "off", the sound become "duller" and less definite. Overall, a not so perfectly tuned piano will sound less satisfying and will loose definition. (definition = every notes can be distinguished clearly)
Even though a normal person can't tell how many beats/sec the piano's unisons are out, this person can still notice quite a little difference in the out of tune unison because of the color change that occurred in the sound. Something like 0.5beat/sec will be noticed because the "quality" of the sound is changed.
Hope that all this make sense.
In general, the beat frequency should be low enough that you can't notice it.
The average lowest range of human hearing is 20Hz, around E0. (Most pianos have A0 as the lowest note, since nearly everyone unimpaired can hear it and it's more "musical"). This means that if the beat frequency is above 20 Hz, you will hear a third note corresponding to that frequency. That's definitely bad.
The "beat" phenomenon still occurs below 20 Hz though, of course. I would approximate one beat every 3 seconds, or a beat frequency of 0.333.... Hz, to be good enough. There's no hard and fast rule, really, since more sensitive people may pick up on a slower beat than others. Once it gets that slow I struggle to notice it, and I certainly wouldn't pick up on it unless I was listening for it. It's harder to get higher strings to stay within 1/3 Hz of each other as well, since tuning adjustments cause a greater frequency change than for lower strings, so you might have to be more lenient with them.
And, unfortunately, you're never going to get around the variation in how your piano sounds. You should try to keep the humidity and temperature consistent (which will help how long the piano stays in tune generally as well), but it will never be perfect.