Hey Growmie,
Switch to the flowering stage as soon as the plant has established a solid root system. In optimal conditions, plants should be kept in their vegetative stage for approximately 60 days. This time period should give the plant the opportunity to maximize yield and acclimatize to growing conditions, and judging from the size of your plants, I'd anytime now would be great. Hopefully, I answered your question and remember "Happy Growing" and "Free your mind one puff at a time"