The refresh data does get the images when the show doesn’t have any. The refresh images is only to update the images on the pages, so it is recommended if you want thumbnails or posters.
The time depends on how long those images have been on TMDB. I usually recommend you to wait an hour after uploading before refreshing, but it can be up to 24 hours for those updates to show up on refresh. Keep in mind that the update only imports the most voted image over there.
Once you refresh a page, the refresh should be complete after a few minutes (depending on the size of the show and how many shows are updating at that moment).