The piano is an instrument that consists of hammers and strings. Like any string, when it is pulled taut, after a while the string will stretch, causing the notes to become out of tune.
To keep your piano in tune, a regular maintenance of 2 tunings a year is suggested. Even if a piano is not played the strings will still stretch, the longer a piano goes without a tuning, the more sessions it will require to get the piano back into a tuned playable condition.
A normal tuning will take around an hour. A piano that hasn't been tuned for years will require 2-3 tunings depending on how badly out of tune it is. These tuning sessions will take longer, usually around an hour and a half or more.
We use a combination method to tune. We use an electronic tuner to tune the middle string (most notes have 3 strings per note) and our ear for the other 2 strings to include the human factor to make the piano sounds better than tuning it all to an electric tuner. This way we know your piano is on perfect pitch.