When the bloom's petals fall off they expose the pod where the seeds are developing. The hollyhock seed pod starts off green and then turns brown. The first time I paid attention to the seed pod I thought it resembled an old coin purse with the string at the top to draw it closed. This seed pod is probably ready to split open and harvest the seeds from. If you look closely, you may be able to spot the outline of the seeds. Although, I prefer to leave the seed pod alone until it opens by itself. When the seeds inside have expanded and the pod has split open, you know the hollyhock seeds are ready to collect. The dark, circular seeds seem to expand to release themselves. Even at this point they don't go very far from the plant, the seeds stick together and are not easily disturbed by wind or rain.
If you are looking to purchase hollyhock seeds, refer to www.edenbrothers.com as they have a selection.