"How often should I have my piano tuned?"
The short answer: "It depends." The industry standard suggested by manufacturers and technicians alike is once per year, even if the piano is never played. Much like an annual visit to your doctor, this keeps deterioration in check, reduces the risk of rust bonds forming, and helps reduce large repairs due to neglect down the road. A piano should be tuned twice a year or even more frequently if played more often or when consistency is a must, like in a church, institution, or an accomplished pianist's home.
"Why won't my piano stay in tune?"
Pianos go out of tune for several reasons. The main reasons are: condition, environment, and time. If the pinblock, bridges, or the strings themselves are compromised in any way, it will affect the instrument's ability to hold the immense tension required for a proper tuning. If the piano is subject to changes in humidity, it will cause the tuning to be altered from swelling or shrinking bridges, and also cause damage from the shift from dryness to excessive moisture. The amount of tension the strings place on the piano is immense, and it is natural to expect the overall pitch to fall over time. This is why whether it is easily detected to be out of tune or not, it is important to have it tuned regularly, in order to maintain the proper tension.
I found a piano for free/really cheap on the internet. Should I get it?
My immediate response to this question is always, "There's no such thing as a free piano." Here's what I mean by that. Usually there's a reason they're wanting to get rid of it, and it's usually because of needed repairs. It is wise to assume that a free or really cheap piano you've found will require you to catch up on maintenance and repairs that have been overlooked. Most likely it will not be at standard pitch (A440) and will require a pitch raise. Expect there to be issues from neglect, including corrosion, mold, and mice infestation. In my experience, be prepared to spend around $400 in repairs and catch-up maintenance for that "free" piano just to get it back to a good starting point. Then expect to spend upwards of $1,000 to get it back to a respectable state of repair. Consider the original cost of the instrument and then compare that to the current price of it. If it was a cheaper piano when new, selling for $5,000, and now it's $50, that amount of depreciation should be enough to consider the instrument wasn't a quality product to begin with, or is in great need of work. It is better to get a good condition, well-maintained, well-manufactured brand of piano and pay more for it than to get a cheap, poorly-manufactured piano and have to spend several hundred or thousands of dollars to make it playable and somewhat enjoyable. At the end of the day, if you don't enjoy playing your piano, it isn't a good piano. It's time to recondition or replace at that point.