Uh throughout the next fiscal year? Big Army gives the board a number to meet for the SFC board that they project will be needed due to retirements, attrition, etc. Depending on what that number is and how many SSG's eventually get promoted that is when you should have a general idea because they get picked up when their number hits on the OML.
That does not mean that is the only way, you still have other SSG's getting out because their contract is over or for other reasons.
I don't know what else to say other than when a slot opens it opens.