If it's out of tune, it's out of tune. It doesn't matter how long. You can hear it after a few days.Would it be true that the more you tune it, the less it will go out of tune? If you wait, the pins will slip a little more, the string will stretch a little more, etc. And when they tune it, it will have to be adjusted farther than if you had it done more frequently.
.... but first I have to remove the rubber wheels the previous owner installed. It raised the piano an inch or two, and makes it a little akward to play on as it is high. I imagine once I raise it, it will disturb the strings.
A new piano requires a minimum of four tunings in the first year. That is one tuning every three months.As the instrument was moved from a seasonally controlled room to a normal environment there will be some settling in to endure over the next several seasons.The heat coming from the window will have an effect on the humidity content of the room. Humidity fluctuations have a greater effect on the wooden structure of the instrument then temperature changes.A while back I got together with a colleague and we did some experiments with humidity effects.Here is a posting from last April on my blog and at the bottom is a link to a photo album of the results of our humidity experiments.Happy viewing.https://silverwoodpianos.blogspot.ca/2011/04/humidity-and-piano-sounding-board.html