From what I have read, the seed pods will get bigger and bigger until they crack open a little, and then eventually open completely and fall apart. you can either harvest the pod once it's cracked open, or if this is your own plant and not one on the corner of your neighbor's front lawn, you can tie a paper bag around the seed pods and just let them open up and drop most of the seeds, harvesting the rest when you're ready. But I'm pretty sure you want to wait until the seed pod is fully mature before harvesting the seeds, at least for quantity.

I have access to several huge datura plants (I'm not sure if they are Inoxia or Strumonium). I have been curious about whether the time that you harvest them is going to effect potency or not. I plan on harvesting some seeds early this summer and harvesting some later in the fall and comparing them.