Don't know the precise answer, but here's what happened to me.
Active duty on flight status in 1980s, with Top Secret clearance. Seven year break. Joined the drilling reserves in the 1990s, and had to fill out all the paperwork for a new Secret clearance. Had to renew that 8.5 years later, still at just Secret level, when recalled to active duty from the drilling reserves and deployed OCONUS.
It might also depend on the similarity (or not) between your military clearance, and the specific rationale for your civilian clearance. For instance, I don't think a generic military Secret clearance will let you work for State, Energy, CIA, etc, without their own added particulars. It should, however, make it go faster. Ooops, there I go, applying logic to the USG.