every inning
Shape your cue tip after every inning, more often if you miscue. Use the cue repairmen's friend, a Willard tool, to shape with. Always carry spare tips with you, a new tip might not last out a long session.
Wait a minute, I am not selling or installing tips anymore! That being the case, with the exception of a really bad quality tip, my old friend TD873 and others saying the same thing are correct. The tip will largely tell you how it wants to be shaped for your play. Clean it up now and then if needed, it probably won't be. Before play use a brad tool or similar to roll firmly across the surface of your tip. Don't let it slip, you are just rolling dents into the leather, not cutting it.
High tips and low tips don't play the same so I advise changing tips with them still fairly tall. That isn't to make a buck, once you get used to the hit of a low tip then all tips will seem different, even the brand new tips of the same brand.
I cut up a small handful of tips when I tried to maintain a dime configuration for awhile. My style of play rarely needs a dime and once I quit being hard headed my milk duds last almost forever.
All tips aren't created equal. Invest under twenty bucks at harbor freight for a set of scales. Weigh the tips you buy or try to talk the repairman into letting you weigh his tips in stock.
When I weighed my box of Elkmasters before dudding them the weight was in four distinct ranges. About ten of them were very very light. There were two distinct sets in the midrange, and four very heavy tips. I dudded the midrange tips, tossed the light ones without too much pain at less than fifty cents apiece, and saved the heavy ones to play with on personal sticks and see if they were special. Lost them in a storm so I'll never know.
I try to avoid grinding tips to shape them, this can destroy a layered tip sometimes. Best results I have found is with the premium utility blades and then using an angled block to get the top bevel very near to ninety degrees to the surface of the tip. This works much better than the usual negative rake without being grabby. When placing the side of a blade on the usual flat block it creates a negative cutting angle trying to have the tip push off of the blade. I also mark the blade every time I shape a tip. The premium blades last longer but I still follow my old protocol of transferring a blade to another holder for general use only after shaping three tips.
I realize that I went a bit beyond the question but how tips are shaped definitely impacts how often they are shaped. If you have a cue repairman at your hall having him reshape the tip if needed is probably worthwhile. Much better to cut leather away than to grind it away.
Hu