I simply answered the OP's question... which was how long..
If you want to know the requirements..
It's something like 50 ranked rounds played AS A MASTER, meaning not total ranked rounds played, but the number of rounds played SINCE becoming a Master.
AND
An average lower than 63, which means 63.XX is not good enough, it needs to be 62.99 or lower.
Once you meet BOTH of those requirements, you will move up.