Certified GKH Gardening Expert
This will, largely, depend on the cultivar of Hibiscus that you have.
I would start them indoors at least 6 weeks before the last frost date in your area, if these are a hardy type. For better results, start them several months early and plant them once they develop true woody stems. For the tropical type, it isn't recommended to start them from seed.
This article will help you to make germination faster: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-propagation.htm