They will turn dark, lose the greenish shade and pop out easily. After its mature the part that holds the seed to plant dries out and seeds become separate from plant itself. This is when they are ready. It takes about 6 weeks, but some seeds mature much earlier than others. At 6 weeks almost all should be ready. You should be easily find and pop out finished seeds at this point. If you see that there are many that are not ready, then let them stay for a while more.