If you've changed your ID card to reflect your altered appearance, as you should, I'd guess your DO may well advise that it should be retained for a minimum period. There's nothing in BR3 Article 3818 (http://www.royalnavy.mod.uk/News-an...ents/Naval Publications/BR 3/br3book/ch38.pdf )to state there are minimum timescales, but I'd guess it's a case of applying a little common sense and not changing one's mind on a whim.
Logically if you cannot get a faceseal when wearing BA or a respirator, then you have good reason to shave off.
I have a beard and had a cause to shave it off about a month ago so that I could have my GSR tested in the special RTF tent, it grew back in about a week or so but I didn't need to get special permission to shave it off/grow it back.
AFAIK there is no minimum timescale for having a beard.
When you do shave-off your SET leave a magnificent womb-brush and sideboards like Del Piero and see how long you can wander round your ship/boat/establishment before being hauled up. I once wandered round barracks for a full day sporting a WWII black cap and no fekker took the slightest, grog was in then if that's of any relevance?