How often should my piano be serviced?

Are you talking about an acoustic piano? I guess yes.

If you ask a piano tuner, they will tell you 'every 3 months'.
I often leave mine for a year before a tune - but only because I am so busy I can never get round to it.

If your piano is old though - it may need to be handled less (once you get it in good condition). They get more delicate as they get older.

Very good.