Thank you for contacting the UVM Master Gardner Helpline with your question.

To be up front I’m not an expert on hibiscus, but I found some information on other extension service web sites for you.  Pruning can be done in late winter/early spring before the early growth starts. If you do it later, you could prune off the bud sets and, therefore, not get as many flowers that year.

They say dividing can be done in the fall when the plant can also be pruned back to make it a more manageable size or in the spring. You might not get flowers for a few years after you divide it.

Here are my sources:

The University of Arkansas Extension recommends pruning in late winter early spring. The flower buds set off new growth. Recommendation is to prune it back 1/3 to 1/2: https://www.uaex.edu/yard-garden/in-the-garden/reference-desk/tropicals/tropical-hibiscus.aspx

Specifically, on Feb 3, 2018, they answered a question about leggy plants with this response: 

Question: I brought a twisted trunk Tropical Hibiscus into sunroom when the weather turned cool. I have had a terrible time with aphids, over a period of time I have sprayed with dawn liquid & water (four times) I still saw aphids so I stripped off all leaves & sprayed again. Did I kill it?

Answer: I doubt you killed it by removing all the leaves, but it sure isn’t happy. I would go ahead and cut the plant back by half. Tropical hibiscus plants bloom on the new growth, and if you keep a plant from year to year, it needs to be repotted and cut back severely before new growth kicks in the following spring. Since it doesn’t have leaves, now is the time to do so. If you allow it to start leafing out on the existing branches, it is going to be weak and leggy and you will have less flowers this summer. After you cut it back, you can take the plant into your bathtub and run the shower over it with tepid water, making sure the water can drain freely. Then move it into a bright, sunny location and see if it doesn’t begin to leaf out. Aphids are notoriously bad swimmers and a nice shower now and then, could do the trick. If you need to make your own insecticidal soap again, try a teaspoon of Murphy’s Oil Soap per gallon of water.

The link below has a lot of information about how to grow your hibiscus plant, including information about pruning which they recommend doing in late winter:  https://extension.umn.edu/house-plants/hibiscus

This link also contains recommendations about growing and pruning:  https://www.lsuagcenter.com/topics/lawn_garden/ornamentals/trees_shrubs/growing-the-tropical-hibiscus-in-louisiana

The Illinois Extension recommends dividing in the spring:  https://extension.illinois.edu/blogs/rhonda-ferrees-ilriverhort/2017-07-28-huge-hibiscus-flowers-are-garden-standout

The University of Minnesota provides this information about dividing plants:  https://extension.umn.edu/planting-and-growing-guides/dividing-perennials

In their dividing plants spreadsheet, they list that dividing can be done in the fall or spring, but the fall might be easier because the Rose Mallow (perennial hibiscus) is slow to send up shoots in the spring.

I hope this information is helpful. If you need more information, please feel free to reach out.

Laura, UVM Master Gardner Helpline Volunteer