Seeds need 4-5 weeks from pollination to mature. Easiest way to tell if they are mature is to find one and remove it from the flower with a small pointed object like a needle or skewer. If the seeds are pale/greenish/white they will need longer. Once you are able to find 3 or more mature, shiny brown seeds, you can confidently harvest the flowers. Often the calyxes containing the seeds will split open, revealing the seeds and letting you see the colour of the seed. The seeds when mature will be shiny and brown and resemble the seeds you buy.
Often it is best to pollinate some of the lower and smaller branches only, as you can then harvest the top of the plant when ready and leave the lower/smaller branches to finish seed maturation.
You will need to be careful with your new seeds - if your "mother" plant was a true hermaphrodite with many male flowers, chances are so will be your new seeds. If however the pollen came from only a few male flowers here and there, your seeds may be reliable and could be female too. Hope this helps... Organoman.