In the 55 plus years I have been involved in the piano industry one of the most asked questions is" when do I know it's time for a different piano"
My answer is always the same. You will feel it in your musical senses. As you progress through the improving stages of playing you will almost instinctively know when it's time to move ahead.
Let’s say you are practicing as much as ever but have reached a plateau and are not seeming to get better as quickly as you have in the past and you can’t figure out why.
You’re not enjoying playing as much as you normally do. You feel there is something missing in your performance even though you are not making mistakes and it sounds alright.
Or you feel that the piano is not expressing the effort and skill you’re putting into it, and you’re frustrated rather than satisfied with your performance.
These sometimes-subtle triggers will tell you it's time to move up to an instrument that makes you happy and satisfied with your playing again.
The one unchanging rule with music is you can only play as good as your instrument will allow you. An inadequate instrument makes for a much more difficult path to go where you want to be as a player.
Get the best piano you can and get the best from your efforts.
It is a great investment in your reason for playing.