Bell Peppers take around 6 weeks to mature and then it can take another few weeks for the peppers to turn from green to red.
So while the peppers are okay to be picked after 6 weeks it will take around 8 to 9 weeks to see your green peppers turn red.
You can still pick the peppers when green and eat them while green but if you want the peppers to turn red give them at least 8 weeks and let them turn red while on the plant.