The longer a piano goes untuned the harder it is to get it back into shape, but we do it all the time. If the strings sit for too long at the wrong pitch it can make the tone a little more unstable than it would have been. Also, strings can rust and get “stuck” at various friction points, increasing the possibility that they may break when stretched.
Neglected pianos can develop sticky keys, or become the victims or insect or rodent infestations. Still, we see pianos every day that have not been serviced for years, and usually have very good results making them playable again. We usually recommend a follow-up tuning after three months, but after that your piano will usually be fine on a minimum maintenance schedule.