They are in 2 gallon pots right now. When should I...

Are you sure you want to transplant? In coco you can get away with smaller pots and 2 gallon sounds perfect for your light wattage, unless it's a mistake in the information and it is actually soil. Also, the missing information we need is not so much the height, but the surface area. Let me explain, 300 watts of LED lights should cover an area of 2.5x2.5 to 3x3 sq ft (6 to 9 sq ft, which means 50 to 30 watts per sq ft); height wise your lights won't penetrate deeper than 12 inches, so having a 6 ft plant is worthless, since only the top 12 inches would produce significant buds. I would veg 5 weeks or 6 weeks and heavily train the plant to fill the whole area but keeping the height at 12 inches max (TOP/FIM, LST or SCROG are your best friends here) and then think of switching to 12/12; the idea is that the final height is 2 ft tall max. That been said if you want to transplant anyway 2 gallon pots of coco will last you the whole veg phase, transplant a few days before switching to flower, if you transplant now you run the risk the roots will not hold all the media and the root zone could be damaged.