2nd grow - advice please

high folks....this is only my second grow and im a little stuck at the moment...i have been growing 2 plants cheaply for amount 3 months now,they are in multipurpose compost/get watered (which contains miracle grow) every 2 to 3 days and were sitting under 7 household cfl bulbs (20 watts each similar to a 100 watt conventional bulb/1150 lumens)which i find are brill for vegging but due to them being in the wrong light spectrum they are a bit slow encouraging bud growth.i have just changed the 7 cfl's for a 300 watt eco cfl grow light with reflector (2700 lumen flowering).

the question i have is : how do i know when the plants are ready for harvesting? the smaller plant is budding and the larger plant is a week into flowering..any help appreciated please.