When they're carbed. Carbonation usually takes a minimum 3 weeks when the beer is at 70 degree for normally gravity beers (some take longer, some may take less time). After 3 weeks, take two bottles, one from each case, chill them for a minimum 48 hours, and try them, if they are carbed and conditioned (tasting how they should) then you can't start chilling and enjoying....If not, try again in a week or two....
It's not a precise process, each beer is different.