Ideally, your piano should be tuned at least every 6 months for stability reasons. Strings cause tension and the piano is put under a big strain. Strings gradually relax causing the whole piano to go out of tune.

There is a standard pitch set for piano tuning - a 440, called ‘concert pitch’. The longer the piano has been left untuned for any reason the bigger is the difference of strain to get back to the concert pitch, requiring more time for body and strings to settle. Pianos, tuned on 6 to 12 months basis can be tuned normally in one tuning session, but if the interval between tunings has been left much longer, there could be more sessions necessary for settling your piano.

Some old pianos can not be tuned to concert pitch either having weak tuning pins or construction. To make such pianos tune-able one needs to look at each case individually. Solving the problem might be just a change of tuning pins or in some cases leaving the pitch below a-440. The last is not recommended when the piano is used to teach children.

You should also take in consideration that if the piano is below the concert pitch it cannot be used along with other instruments.

Once you are replacing tuning pins why not to replace severely rusted treble strings at the same time and maybe bass strings as an option? That would give totally fresh life to the sound of your piano.

Unfortunately few pianos will have to end their mission and be disposed of.