All of mine are optimised, it's seamless and just works away at it in the background when you're not using your phone, so takes a little while.
Unfortunately I can't remember how much space they occupied on my iPhone before, but I can tell you that my 6,158 photos and 179 videos occupy 9.4GB of space on my iPhone after switching to optimised. And my iCloud Photo Library, which has the exact same content as that which is on my phone (albeit the iCloud library has the full resolution material) is 49GB.
So if that equates as it would appear, I'm saving 40GB of space on my iPhone.
I should say that not all of the photos are from the iPhone. Some are from my DSLR and so have a much larger file size. But because I added them to the Photos app on my Mac they are also available to me (in optimised format) on my iPhone.