How do I cut a hibiscus?

Hi there, I cut my hydrangea back every fall about this time. It really depends on your zone and your weather. As long as the bush is dormant you're good to go. Here's a link for some more information on how to prune hibiscus:

https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm

I generally wait until almost all the leaves have fallen off or the plant looks 'dead', then cut as much of the stems I can. Come early spring they will came back with a vengeance, over twice its size & fullness.

For the most part, hibiscus plants can be lightly pruned in late summer or early fall, but no hibiscus pruning should be done during late fall or winter. One of the downsides to waiting later in the season to prune is that plants may not develop as many branches, and they will put out fewer blooms

To reduce the risk of removing these buds, prune just as the flowers begin to fade. Often, the earlier you get it done after bloom, the quicker the shrub can recover, producing more and larger blooms next season. Don't prune these hydrangeas to the ground in late fall. Doing so removes all of next year's flower buds.

Wait a bit to prune hydrangeas: https://thekitchengarten.com/why-and-how-to-prune-hydrangeas/