From: St. Petersburg, Florida, USA
I often don't even leave a regiment.
If my opponent were to retake, say, Makin in August 1943 (or later more likely) and kill a couple of USAAF base forces or whatever I put there... well, go for it. Such an operation would leave me with great openings elsewhere because he's going to have to invest in air cover for the invasion - just a single squadron of Dauntlesses would have a field day, even if they didn't stop the invasion completely. Don't even need to have a second island with an airfield, really. Unless I'm overstacked or the airfield was not built up very far, I'm not terribly concerned by bombardment runs by negligible IJN forces. If the bombardment TFs are non-negligible, again - that means a door or window has opened elsewhere.
I do pretty much the same. I might leave a regiment on the main base atoll of an island group, but the others get cleared of occupants as I need to martial the seebees ad special seebees and port maintenance base forces and Marine defense battalions to move them forward to the next group of atolls.
I'm Jan '45 now and I have almost completely denuded the Marshall and Gilberts over the past two months getting all those units back to Pearl and prepping for points in the PI.