More often than not, when people ask me whether it's time to replace a part on their bike they've come in about 379.37259 miles sooner than I would have started thinking about doing it. This seems to be the case with tires, brake pads and chains, the most common replacement items.
As you might expect, people who come in declaring everything is in tip-top shape but they just want the bike looked at have completely toasted every replaceable part. But this isn't about them.
When faced with an almost worn out part I used to tell the rider to take off for a few days or weeks, depending on the amount they ride, and bring the bike back when the last value had been extracted from the part or parts in question. That never worked. Now I just replace things when they are here. Otherwise the timely repair won't happen and more expensive things might get worn out or damaged before the customer finds time in the schedule or is forced by a serious malfunction to bring the bike for service.
Today's subject wanted brake pads. The rear set was pretty low but the front pads are awkward: There's about half a pad left. But when am I going to see the bike again? This guy will definitely ride his carbon fiber Specialized Roubaix with its 700X23 tires down dirt roads and across muddy fields. He himself is rugged and durable. His mind directs his body and the bike gets dragged along for the ride.
Most people wear rear pads faster than front pads. Front brake phobia strikes to varying degrees, from the Grade 4 sufferer who absolutely never squeezes that lever to the Grade 1 type who uses it moderately but assumes that an occasional endo is inevitable punishment for that rashness.
When a bike is checked in for repair I try to inspect as much as possible in front of the customer so I can get a clear decision from them without having to play phone tag. When the customer is in a hurry I have to write down their comments, try to get clearance for probable extras and deal with other things as I discover them.