The right time to buy a bowling ball is when you just want something new to use. You should buy a new bowling ball when your other bowling balls on not working like you want them too and they get cracked or chip.
Now with your older bowling balls, you can take them to a bowling shop to change the surface or clean them up so they can work better. What I like to do is get my bowling balls deep clean at the bowling shop. By getting you bowling balls deep cleaned, the ball will work like when it came out the box,
For me, I tend to buy new bowling ball when I just want something new to use. Sometimes your bowling balls hit the wall or they can’t do any more than it can now. When the lanes break down so bad that the bowling ball that you have do not work than may it’s time to buy something that is build for dry lanes. Some days your older bowling balls will just not work and need something else.
Over time if you don’t wipe off the oil of your ball, then that oil will decrease the hook of your ball or make it work less. Only buy a new ball if you really need it. If your older bowling balls are working than I would not replace them any buy anymore.
I see too many people just buy new bowling balls and they has not mastered the ball that they have. Once you mastered your older bowling equipment and had it for a while then yeah maybe it’s time for a new ball but I still use a bowling ball that I have had for the past 4 years and still going strong.
I would clean up your older bowling equipment and see how they roll before buying a new ball. This is the cleaner I use it get my bowling ball clean.