How to delete photos to create space in iCloud

I got a notice that I have less than 1GB left of my 50 GB iCloud storage. I checked my iCloud storage and it appears that 33. 2GB is used by photos and my iPhone backup is using 12.1 GB. I noticed that I had over 4000 photos and quite a few videos on my Camera roll so I set about deleting. After deleting about 2000 photos and a videos from Camera Roll yesterday and today, the iCloud storage numbers remain exactly the same. I have switched the phone completely OFF and On again. How long does it take to register changes in storage? I also have iPad and iMac computer. The iPad seems to take up a relatively small amount of space and only has about 400 photos in the camera roll.