Quick question regarding vacancies and new hires -- do unfilled vacancies stack with regard to new hire slots?

In other words, for a given BES, if there's 10 unfilled last month, 10 unfilled this month, and 10 unfilled next month, does that translate to 30 new hire slots? Or is it only 10?

I'm just trying to get a sense of whether certain BESes are growing or shrinking with each successive bid. The staffing guy also said that eventually unfilleds need to be "refreshed". Is there are a hard and fast expiration date on them, or is it just kind of fuzzy math that's completely internal to the company?