Where you live, it's a little late in the year to force new growth, it won't have time to harden off before winter. I'd be inclined to leave it till Spring, after its flowered (if it does) and prune it then. If you don't get flowers because its too small, prune it beginning of May.
This plant gets rather large, though can be restricted by pruning - will need a very large container as it matures.

Buddleja globosa flowers on old wood so you prune after flowering, which is May June. So I would not prune it at all until it has flowered. To be honest they do get to be a very big shrub. Until this winter knocked them down our two were well over 12 feet tall and as wide.