Are You referring to the time to change the tip for fear of cracking the ferrule, or just time to change in general?
Although this isn't My current thinking, I felt this at one time, and have many people tell me some tips play better when they are too low, and in need of changing. I have found this to be true with the style of play I use to use, but now days I never let the tip get very low on My personal cues. They tend to last Me alot longer now anyhow, because I don't use radius shapers anymore, just a tip-pik to hold chalk.
I really eyeball them Myself, and most customers decide for theirselves when they want them changed. I can only warn Them ahead of time, but the rest is up to Them.
1/16 to Me on the sides not the middle is a good time to change out to be safe. I really put more emphases on the sides then the middle, but I can see where others may have the oposite thinking. I guess IMO that may rely more on the players style, how hard they hit, and how far & often they hit offcenter of the cueball. Ofcoarse breaking with a low tip only increases the chance of cracking a ferrule, so that's something else I may would take into consideration.
Greg