I realize that we all have to start somewhere, and I usually try to put myself in the individual's shoes, who is asking the question before I reply/answer. But, there are also some things is this area that I feel strongly about..... so this answer is a mix of looking at it from a "newbie's"
position.....and my own, as someone who's been doing it, and selling knives for more than 3 decades.
My opinion is very similar to Chris's, but I would put it this way..... When you are confident enough in your entire skill set to... 1. Mark each knife with your NAME...not some obscure logo or initials, and... 2. Take someone's money for your knives, AND along with taking money, offer a full warranty/guarantee, then I feel like a person can call themselves a "Knifemaker".
These days the lines have been blurred beyond recognition. There are people out there who make beautiful KSOs (Knife Shaped Objects)..... but they know NOTHING about steel, heat treating, or anything else that is the actual "heart and soul" of a knife. They are selling these KSOs for big bucks, and buyers are being ripped off left and right.
I try to keep an ear to the ground on issues like this, and based on what I'm hearing, and have been for some time, there are far more people that are suspicious of people who call themselves "Knifemaker", or "Bladesmith" then ever before. I actually have more people then ever who will ask about my MS rating, what it means, and what was required to achieve it, then ever before.
Most are those who contact me to ask advice on a KSO they've purchased, and are unhappy with, saying that although it's a "beautiful" piece, it's a terrible cutting tool...and my advice is that they need to be contacting the seller, and insisting that they, in one way or another, make it right, and just as often they don't know where to start because the knife has that "obscure logo or initials" on it.
Not only do those putting out KSOs impact the custom knife market, but they also cast real/true "Custom Knifemakers" in a bad light, and that is the part that I take personal..... some might say it's none of my business, but I choose to make it my business..... to protect and defend the industry that I have been a part of, for longer then most of the KSO makers have been on this earth.
Let me clarify..... First, I know that what I said about marking a knife might raise the hackles of those who use a logo or something other then their names, but, I personally believe, and have seen it more then enough times, to believe that many who use a logo or just initials do so, at least in part, to "hide" behind it, fulling knowing that their product isn't up to snuff, and that they will be far more difficult to locate should a buyer have an issue. Initials are generally something you see from very inexperienced makers, who often don't know any better, but again, if I see someone putting initials on a knife that is "for sale", that usually reflects that their skill set and/or knowledge is NOT where it should be, to be selling their knives. When you apply YOUR NAME to a knife, it if far more personal, and it is taking far more responsibility then could ever be with a logo or initials.
The part about taking someone's money for your work is pretty self evident, but when you add/place a warranty/guarantee on it, and in conjunction with an actual NAME being on the knife...... that means you take full responsibility for that knife, and for any and all issues/failures that may occur.
So when do I think you should call yourself a knifemaker? When you feel confident enough to place your NAME on a knife, and you are fully willing to warranty said knife/knives, for repair, or refund for any problems that related to the build/function of that knife for it's intended purpose(s).