How much space will iCloud Photo Library use on my devices?

I am considering to use iCloud Photo Library to store a growing number of family photo albums, shared to web or set of AppleIDs.

I dont need to keep original photos on iDevices for 99.9% of photos.

How much space will be used on the photo-originating set of devices - what (roughly) is the function from the original size and number of photos, device storage space available, any other variables?