If you mean the shrub Lavatera, the one that has a woody base, the time to prune that (hard) is in late March (in the south) or early April (further north), always considering what the weather's like in any particular year. It usually flowers practically continuously throughout summer, so don't cut back hard, but a bit of trimming here and there won't do any harm.

Thank you Bamboo.I did mean the shrub.It just gets so huge but will heed your advice.

I used to cut mine to the ground in spring, or at least, right back to the lumpy bare wood at the base, and it still got to 9 feet by end of summer.