I don't think it's time based.
Sargeant Major (either Warrant Officer first or second class) is an Officer by Warrant, as opposed to a Commisioned Officer which obviously has a commission. It's true though that a WOI or WOII would have come through the ranks, not the sausage factory, to earn the title Sir.

It's all confusing. Our RSM insisted on calling him Sir even in the Mess..but the Commissioned Officers would insist on being called Peter or Rupert or whatever.
( Piper Alpha is being a grumpy northerm bastard on:, Sun 9 May 2010, 17:23, Reply)