I think as others have mentioned how you define a McDojo is kind of important. If you see a McDojo simply as a martial arts school primarily concerned with making money than, I don't necessarily think that a McDojo is always a bad thing. A school's (and it's owner's) main motivation can be to make money and still teach legitimate martial arts of good quality. If you define McDojo as a scam or a school that's poor quality then that's another story.
I realize the OP is kind of old, but they mentioned in a subsequent post that it was mentioned that they were told they could get a back belt in two years by another student. I supposed whether this is a red flag of some sort depends on the context of the comment. If it was, "Hey man, you keep coming and working hard, you could get your black belt in about two years," then I don't really see that as a problem. As others have mentioned a black belt in TKD in 2-3 years isn't unusual and the other student may have just been trying to be nice and motivational. It the comment was more, "Hey man you can be a black belt in two years if you sign this contract and/or buy X,Y or Z," then I see that as an issue.
There are plenty of schools in my area that I would consider McDojos. They have multiple locations and they have a slightly corporate or franchise feel to them. They do legitimately teach martial arts, however. It's not my cup of tea to train there, but I don't begrudge people who do enjoy their training. I imagine some would see my Dojo as McDojo as it is my head instructors primary source of income. He is concerned with both with retaining and bringing in new students on one hand because he loves to teach, but on the other hand, because it helps his bottom line. However, his business practices are fair (at least IMO) and transparent and he's willing to work with people who are having financial problems, especially if they've been a student for any length of time.
On the other end of the spectrum, there is an instructor in the area that has opened and closed a couple schools already (I know of him because he used to rent the same space our current Dojo is in). He claims to teach Kung Fu, but from what I've seen of video of his classes (that he's posted himself) it's a weird mishmash of several different arts, mostly TKD. He posts fairly outrageous claims about how after only a couple classes parents have told him how much better behaved their children are and how he took his white belts to a tournament and beat everyone. And despite only opening this particular iteration of his school a couple months ago, he has what seems to be several highly ranked children as students (I'm not sure how his ranking system works, but the first two belts appear to be white and then yellow, he's posted picture with several students wearing purple belts). By comparison, an instructor at my school opened her own Dojo in a neighboring town and after almost a year her highest ranked student is a Go-Kyu (yellow belt). Now, this school I would call both a McDojo as his primary motivation appears to be making money, and I also think pretty much everything about his school and what he teaches is fishy.
So, I don't think that a school saying, "If you work hard and train regularly, you can possibly reach black belt in X amount of time," is in and of itself a bad thing. I think that if a place is guaranteeing you a black belt in a certain time span, or they're making far out claims about what they teach or how effective what they teach is, then I would run. If the instructor makes seem like he is a holder of some super special secret martial arts knowledge, then run. If the school claims that only they and no one else teaches truly effective martial arts or that they have the secret to learning martial arts faster, then I would run. If you just think it's a school that's trying to make money off of you, but you enjoy it and what they teach seems legit without outrageous claims, then you have to evaluate how you feel about that. I personally don't see a problem with a school trying to make money and wouldn't really have a problem attending class there and long as their business practices where fair and upfront.