I am a trained concert tuner and was taught by technicians that worked for Horowitz and Rubenstein that  the piano is to be as in tune at the end of the concerto as it was at the beginning. How often the piano is played should have the LEAST effect on the tuning.

Piano manufacturers recommend tuning the instrument at least twice a year. However, there are times when pianos need additional tuning. The farther the pitch needs to be corrected the shorter amount of time it will stay in tune

A  piano is a work of art. Therefore, to treat it rough, carelessly or negligent it is to commit a crime against a beautiful piece of craftsmanship. To pay money for a fine piano and then allow it to go to ruin for lack of expert care is not merely aesthetically wrong it is bad economics. If a piano is neglected, if it be allowed to go through from one season to another, say, from Spring to Winter, without tuning, it will probably, at the end of that time, be considerably lower in pitch than it was originally. It will have gone through a rise, followed by a fall, and the fall will be greater than the first rise.

No matter what any salesman may say, no matter how well the piano may be made, no matter, in fact, what the physical circumstances, the price or weather conditions may be, there is no such thing as a piano standing month after month in tune. The better the piano, the more frequent and careful tuning it should have.