If you're declaring to your taskers that you have the capability to do night decks and wets, then the once a month currency that you suggest seems about right. It may be that months or even years pass before a genuinely challenging job crops up (depends where you're based I suppose) but if you're saying to the authority that you can do it, they'll expect no less.
Along with the mountains, these are the two areas of SAR where there's the greatest risk of it all going wrong. To get it right when the moment arrives, the whole crew's skills have to be honed to perfection - especially with something like night wets if the winch operator has control of the aircraft.