|When is the best time and how much can I trim my shrub without affecting it from regrowing or filling itself back in?|
I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
Shrubs are amazingly able to regrow from even a very severe pruning. The exception are junipers or other needled evergreens which will not regrow if you trim them back past the part of the branches with living needles. The degree to which you should cut other shrubs back depends on your goal. If you want them to just be fuller, then shear them back just a little regularly and they will branch more and create a very dense foliaged plant. If the goal is to make them smaller I suggest you cut them back to a little lower than you want them to be so as they regrow you can shear them several times cutting a little farther out each time to create a dense shrub but still not have them be larger than you want.
Severe pruning is best done in later winter to early spring but can be done on through summer. Don't cut them back severely after about mid August or you'll just promote a lot of late season growth that will be more subject to winter damage.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!