After the flower has finished blooming for the season,
a large pod will start to form on the stalk where the flower was.
Fully developed it is the size of a walnut and will be clearly visible.
When the pod turns brown its time to pick it off.
There are approximately72 seeds in each pod.
If you're growing tropical hibiscus, they often have to be hand pollinated if you want seeds. I grow a whole bunch of different tropical hibiscus and have only gotten seed pods twice (I don't take the time to hand-pollinate). So if you want seeds you'll probably have to help things along a bit. Here's a site that has a picture of a seedpod, as well as some info on how to pollinate if you want to try that approach http://www.trop-hibiscus.com/hibseed.html