It will charge as soon as the engine is running, in any gear. At idle though the charge rate can be quite low, in fact if you have a lot of electrical stuff on (lights, aircon, heated seats, etc.) the battery may even be discharging. In the 'old days' it was common to have a 'centre zero' ammeter in the instrument cluster so you could actually see whether the battery was charging or discharging, and at what rate. Modern alternators and batteries are better so there's less need to know nowadays.