When is storage freed after Photos app uploads to iCloud (via "Optimize mac storage" setting)?

I've had "Optimize Mac Storage" setting enabled for a long while on my MacBook Air (see that setting is selected in the below screenshot). However, I recently imported ~20gb of photos to my Photos app. It seems as if the photos have successfully uploaded to iCloud, but it doesn't seem like much, if any space, has been freed on my MacBook yet. My Mac is low on harddrive space—only 12gb out of 500gb left. When will I see the additional hard drive space, supposedly freed by the Photos app purging the originals? Is there a nightly process that runs? Is it when I restart?