I only do an hour a month, just so I don't get put on the 'inactive' list. Apparently there's a big push (from the GB) to chase up inactive ones. (This I heard from my elder hubby's own lips after a meeting with the CO. I know as an elder's wife I am going to be in the sights of the CO which is making me nervous.
I've thought about this question a lot and have thought all I need to say if asked why I'm doing so little is just, 'I have my reasons but would like to keep it to myself, if you don't mind'. I think the trick is to be firm and assertive without being rude. It works well in theory... I have yet to put it into practice!