Developed benign migratory glossitis months ago, its pretty much constant. usually how long does it last?

Geographic tongue: Geographic tongue is the other name for benign migratory glossitis. It may come and go or change very quickly in days, weeks, or months. It may last up to a year. It may reappear later.

Depends...: Your condition may last indefinitely. It's widely variable among individuals and most often it will tend to increase and decrease in severity. Occasionally it will spontaneously resolve. If it is bothersome, it can be treated with a topical steroid medication. This will treat the symptoms but will not cure the condition.