Remember that all musicians ( GREAT and NOT SO GREAT) will get what I call LAZY EARS if you let your piano go too long between TUNE UPS!  For each piano the length of time varies on if it is too long or not! For instance some pianos being played a lot will need a tune up every 6 months! Then there are some pianos that once a year is all it will need! Whatever the case remember that as it goes OUT OF TUNE  then your ears get LAZY and USED to it going bad!  So GET IT TUNED before it BOTHERS YOU as that should be the RULE for ALL who want their PIANO SOUNDING ITS BEST ALL THE TIME!!!!!