With me, it changes. There are some times of the year when I don't feel like playing piano very much, so I don't. Then, there are other times that I do, like now. Generally, I complete about maybe..... 10 or so pieces a year, sometimes less.
Usually, in October, me and my teacher choose 4 pieces to perfect for Certificate of Merit (this year I am doing level eight). I work on those pieces until the test, which usually takes place around February. Sometimes I might finish one piece early, so then I'll start a new piece for CM. I usually do that once or twice. Then, after the test, I just work on different pieces until it's time to start preparing CM pieces again. Some years I get kind of tired of piano after CM, so I only learn 3 or 4 pieces. Other times, when I'm more enthusiastic about piano, I'll learn as many as 5 to 8 pieces.
So basically, when I practice a good amount, around 45 mins - 1 hour every day, I can get through about 4 pieces in 4-5 months. When I'm less enthusiastic about piano, then it takes longer as I don't practice as much.