I don't think it will take too long. I pruned one quite sharply a few years ago and the following spring I still got some lovely red and leafy branches. In fact, I turned mine into a tree by cutting off all the lower branches. It also flowered quite profusely after that.

I keep ours as a bush at about five feet. It recovers quite quickly. Its only the new leaves that are red so pruning it back will encourage more. Ashamed to say I cut it back when I think about it at any time of year and it doesn't seem to mind at all.

You can prune Red Robin as often as you like, and when you like from spring to end August. It will produce new red growth after each prune.