In most cases, downtime is extremely minimal. Our typical workflow is to run changes on a staging site or locally first, make sure things work, and then push them live. In terms of actual downtime, your live site should not be offline for any more than just a few moments.
On base level Wordpress maintenance plans, our monthly updates typically do not result in any downtime at all.
If we're fulfilling multiple hours of work on your site, it depends on the work involved. In most cases, downtime still won't be longer than just a few minutes. If we know there's going to be some time where you can't access the site, we'll warn you ahead of time.
At Curious Minds Media, peace of mind is our top priority.