How much space does iCloud Photos optimization save?

[BrettDS]

Where do I turn on optimization? I can't find it.

Go to settings, iCloud, photos. You need to turn on iCloud photo library first. Apparently that option still isn't available to everyone, so if you don't have it you may still need to wait. If you do have it, then the option to optimize space will appear below it when it's enabled.

So let's say I have it set to Optimize, and I download a photo to view or send to someone. Will the HQ photo stay local on the device from then on, or will it revert back to the optimized version if not accessed within a certain period of time, say 24 hours or so?

Once you look at it does it then save a copy on the phone kind of like iTunes in The Cloud or ITunes Match?

Once you look at or share a picture then it does retain the picture on your device for some period of time, but not indefinitely. I haven't managed to figure out how long it stays available.