Our household insurance states it must be swept a minimum of once a year for the policy to be valid. If the stove is only used in winter and not year round then once a year should be enough. My chimney sweep puts a recommendation on the jobsheet for when it should be done next.
I've just looked in to this very thing as we have a coal-effect gas fire in our victorian hearth but the effect is somewhat ruined by a constant stream of muck and bullets falling down said chimney. I have to say that we've lived here for years and have NEVER had the chimney swept! The recommended timescale is approximately x 1 per year - but more often if you burn wood (Quarterly when in use).