Perhaps a couple times a year and then once a year after that. Of course the more it is played the more often it will need to be tuned especially if the student is practicing upwards to 2 to 3 hours a day. Once a year is fine if a 30 minute to 45 minute practice schedule is maintained. When buying a used piano, ask about its history from the seller if it is a private sale. Most good tuners provide documentation inside the piano on their card or inside the piano bench.
Touch Look for a piano that has a even action throughout the entire keyboard.
Avoid pianos that have uneven tone or touch, you or your teacher-tuner should be able to determine if the piano has a consistent and even quality throughout. The piano should have a comfortable and even response. The touch is a vital and critical area to assess before you make the final decision. Brand This is an area that has changed dramatically over the years, the manufacturer landscape is quite different then it was 30-40 years ago. Phone up piano tuners or technicians and ask them their opinion about who they feel provide the best pianos.