It depends on where in the world you are practicing judo. Each country has their own rules and regulations on how fast you can progress with your belt exams. In some countries, adults can do their grading [belt exam] every 3 months.
Children tend to have different procedures for belt examination; therefore, the timeline is different too. Find your national federation via the following link and contact them to find out how often they hold belt exams: https://www.ijf.org/countries