A qualified technician can tune a piano in less than 1.5 hours. An appointment to tune your piano is expected to be around 2 hours to allow for unforseen repairs or adjustments and a little banter with you, the piano owner.
Pianos that are very flat or very sharp need what we call a pitch correction. A tuner brings the pitch of the piano up to A440, measured at the A above middle C, in a quick pass to apply the correct bearing on the soundboard. Next they will fine tune the piano. This still may only take 1.5 hours, but can take as much as 2 hours. Unlike a guitar with 6 strings, or a violin with 4, a piano has over 230 strings! Tuning is a skilled art, and requires an extreme amount of concentration.
Don't be surprised if your piano tuner takes less than an hour to tune your piano. An upright piano at pitch often takes me less than an hour, but this is not the standard. If you haven't had the piano tuned in more than a year and your tuner only takes 15-20 minutes, then I would be concerned they weren't completing the job correctly.