I always tell customers that their piano needs to be tuned at least once a year, but here’s a link from the piano technicians guild that explains why:
“Due to the natural elasticity in new piano strings, we recommend that your instrument be tuned two to four times the first year, twice the second year and a minimum of once per year thereafter. Of course, you may choose to have your piano tuned more or less often to satisfy your own personal requirements”.
– Richard, Piano Tuner Sheffield