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Growing broccoli is a test of patience. Once the head begins to form it can take 2-3 weeks before it's ready to harvest. The above picture was taken 2 weeks after the head began to form. It is looking good at this point. You could eat it at this point, but you really would like to wait until it's bigger. But you have to be careful. If at any point you see any signs of flowering or the individual florets start to grow at a different rate that it is time to cut it. You do not want to let the plant flower at all or it will taste bitter. So once the head begins to form you need to be looking at the plant every day to see how it's going.
The good thing about broccoli is after you do make the big decision to harvest the main head, you get to see those small side shoots start to form. Cabbage will do the same eventually, but in the same time you could just plant another one. Happy gardening!