Well, assuming that you can set a temperament, count beat rates, tune unisons beatless, and put the proper amount of stretch in the bass and high treble, 90 minutes is the industry average.
If you've never tuned a piano before, it will take you the next ten years of your life. Tuning is an art form. Many tuners will tell you it takes your first 500 pianos to be a mediocre tuner, and your first 1000 to be considered just okay.
Here's some advice, don't try to tune your own, or anyone else's piano. Pay a tuner to do it right. You going to wind up paying one anyhow when you realize how very difficult tuning a piano really is.