Trimming rockrose plants

I have several rockrose plants which are growing in almost full shade. I know that they should be getting more sun but I want to keep them where they are. Due to lack of sun they have become very leggy on the one hand, but they also are getting largerr than I would like. I want to trim them back but am not sure what is the best time of year to do this or how far back I can cut them. Any adivice would be appreciated.
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