Rose of Sharon is usually applied to Hypericum calycinum, which isn't usually pruned at all, so I'll assume you're using it for Hyericum Hidcote - should be cut back hard in late March/early April, depending on where you live.

Hibiscus syriacus is also known as Rose of Sharon.

Well that's a new one on me, Tugbrethil, never heard it applied to Hibiscus before. Wonder if that's exclusive to the States?

Maybe, but it's pretty universal here, and I've never heard someone from the States refer to Hypericum as Rose of Sharon--it's always "Aaron's Beard" here. Funny about common names!

Yea, pain the proverbial, common names, you've never got any idea what plant people really mean... if you apply it to humans it's more obvious, I mean if I talk about the fat, bald bloke, that could mean any number of men, but if I say his name, then they all know who I mean.