As soon as the blooms turn greenish, you can give it a prune. Or if you get tired of the white blooms, you can start to prune a little. If you are going to do a lot of pruning, it may be best to do it when the plant is dormant in winter and limit the pruning to about 1/3 of the plant per growing season (you can do another third in another year and the remaining 1/3 on the next year).
Pruning normally makes the plant go into "grow mode" and we are close to summer, a time when you do not want to be in grow mode unless your summer temps are not too hot.
The plant in the picture looks fine to me though. It typically gets 6-8' and looks to be under that in the picture. It also gets 6-8' wide. It appears to be close to other plants and may be eventually grow "too much" too close (guesstimate measuring 3' in every direction coming from the base of the stems). Hydrangea Pee Wee would stay more in check (3-4' tall and 3' wide) and away from the other plants.