The only are that I could find that even mentions custom was canon law so I turned there since it has a whole section on it.
Can. 23 Only that custom introduced by a community of the faithful and approved by the legislator according to the norm of the following canons has the force of law.
In other words, for something to be a custom, it has to approved by the person that is capable of approving it. If something is reseved to the Bishop, he is the only one who can approve it. If it can be approved locally, then the pastor can approve it as well.
Can. 24 §1. No custom which is contrary to divine law can obtain the force of law.
§2. A custom contrary to or beyond canon law (praeter ius canonicum) cannot obtain the force of law unless it is reasonable; a custom which is expressly reprobated in the law, however, is not reasonable.
So, you cannot claim that it is the custom of this area to believe that Jesus wasn't really the redeemer. That would be contrary to divine law. Also, you cannot break church law unless it is reasonable. What's reasonable? Let me give an example. In the diocese of Orlando, when a catholic marries a non catholic in a non catholic church, the diocese mandates that a priest or deacon be present to receive the vows. In the Archdiocese of Dubuque, no such mandate exists. Both believe that are faithfully interpreting canon law. There's some leeway in the interpretation of the canon and both dioceses have ways of interepreting it. It seems like they are saying that it's not reasonable to outright break a law but it is reasonable to have difference in application of the law.Can. 25 No custom obtains the force of law unless it has been observed with the intention of introducing a law by a community capable at least of receiving law.
Can. 26 Unless the competent legislator has specifically approved it, a custom contrary to the canon law now in force or one beyond a canonical law (praeter legem canonicam) obtains the force of law only if it has been legitimately observed for thirty continuous and complete years. Only a centenary or immemorial custom, however, can prevail against a canonical law which contains a clause prohibiting future customs.
This canon is very interesting because it provides a time limit. For something to be a custom, it has to have existed for "thirty continuous and complete years." That means you cannon do something for five or ten or even 29 years and say that it's a custom. It only becomes a custom when a community has lived with it for 30 years. This sort of makes sense when you think that, in the course of 30 years, several different leaders will work with the custom. If they all believe it's worth maintaining, then it's definitely not just a fad. It's a real custom.
So, it seems to me, that when we use the term of custom in its ecclesiastical understanding, you have to ask yourself 1. who has the authority to legislate the custom (Rome, diocese, religious community, or local parish?), 2. does it contradict either divine or church law, and 3. how long has the custom existed? Once you answer those questions, then you know if you're dealing with a custom and if your custom is licit.