Any non healing lesion that has persisted with or without treatment for 14 days needs to be biopsied. That period of time can be determined by the patient coming in and saying "it's been there for a couple of weeks" or the dental team noticing it and starting to count it from that point. Many times the patient is unaware of it because the sore/lesion can be painless. So the 14 day period is when most things resolve on their own. Cold sores, apthous ulcers, cheek bites, pizza burns on your tongue, etc. all heal by themselves in 14 days or less. So anything persisting after that length of time deserves further exploration, and a biopsy is a black and white answer to what it is. Antibiotics for these things usually are not productive because so many of them are viral in origin, and of course antibiotics have no impact on viral etiology lesions.

A biopsy is a simple inexpensive, little physical pain, procedure that leave you with an finite answer. The earlier a finite answer, the earlier you can deal with something dangerous, or the earlier you can quit worrying about something benign.

Since you are seeing a dentist, I would add that I am not a big proponent of brush biopsy procedures which some dentists do. They give you back an ambiguous answer, and in your case where you say you have a visible lesion, not just a white spot, they are contraindicated for what is essentially a class two lesion. If it is to be biopsy, see that they refer you to an oral surgeon to get a small piece of it taken and read by a certified oral pathologist.... don't mess round wasting time with a brush biopsy.