Volume. The total number of training sessions you have logged. The IBJJF doesn't set a hard count, but most professors look for consistent volume over time before promoting.
Consistency. Showing up week after week matters more than training hard for a month and disappearing. The IBJJF system rewards practitioners who train regularly across years.
Time in grade. The minimum months you must spend at your current belt before you are eligible for the next one. This is the only requirement that is officially measured.