Keely weely wrote:
When do I cut it though?
If you want to keep it blooming, deadhead (cut individual flowers off) when their petals drop and before their seeds form. If you want to want a more compact plant with more flowers overall, it's a matter of training and you can start your pruning regimen when it is a foot or so high - in Spring. If you prune it back hard now, you might not get a lot of regrowth, depending on when your first frost date is. It would be helpful for you to google to find more info, or check in a library for books on cultivating perennials for diagrams, etc.
But as I tried to say, it is usually used as an airy filler plant rather than a star, so I've never seen anyone do more than deadhead it.