The tonal stability of any piano is affected by the climate, how frequently it's played, how hard it's played and if it's moved from one location to another with a radically different climate. It is therefore recommended that the normal intervals for tuning are about every six months. In certain venues (recording studios, nightclubs) it's even more important to have this done frequently, even monthly(though the studios won't know till the last minute that a tuning is necessary for the benefit of the artist who's performing). But it's our job as piano techs to educate the public about the need for this. For instance, a piano isn't accurately tuned unless it matches the standard concert pitch at 440 cps which means it matches the tuning fork pitch at that particular frequency. If it's too far off from that, we need to do a "pitch raise" tuning which consists of rendering the pitch sharper than 440(basically a "ball park" kind of tuning) and this is followed by a "fine" tuning .(I would add that from a technical standpoint, the strings are then "equalized" in tension across various points starting from the front to rear segments). And with a new piano, a further tuning would quite likely be needed in a few months to stabilize the initial tuning. And with really old pianos, there's the possibility of string breakage.
Less frequently, we run into situations with the piano's pitch being too sharp relative to the concert pitch, this arising from exposure to high humidity or cold weather. But I've seen that pianos being exposed to air conditioning for too long a time will also result in a raised pitch and will stay sharp even while I'm attempting to flatten the pitch to what it should be. Those strings do resist change!