I recommend that a piano be tuned at least once each year. It is more preferable to have this done every six months especially if there is a change in humidity, if you have children taking lessons or if you are a piano teacher. If you are a piano teacher you may want to even do it more often (and its a tax write-off).
Although the weather is fairly consistent in the Las Vegas area the piano is still affected by the climatic changes in the humidity. Changes in humidity result in the swelling and contraction of the wood, resulting in constant increases and decreases of string tension, thereby continually changing the "tuning."
You will receive and maintain the best tone quality from you piano when it is tuned on a consistent basis. When a number of years passed between tuning, the pitch will drop, often far enough that at least two tunings will be necessary to restore the pitch to the proper level of concert A-440 pitch.
It is sometimes my practice to bring the pitch back up to normal with an initial pitch raise tuning then returning in a few days with a fine tune. Using a device known as a "Electronic Tuning Device" (ETD) helps accomplish the pitch raise very well then I can come back to do the fine tune with confidence.
I will be glad to work with you in setting up a schedule. I keep a record of each tuning appointment in my computer database, and I generally e-mail reminders or call when a year is approaching.