How much extra battery life can be expected when Optimized Charging feature is active?

Are you absolutely certain your phone isn't charging up to 80% during the night, then before you normally wake in the morning, topping it to 100%? This is what optimized battery charging does. It does not hold at 80%. It is supposed to hold at 80%, then before you wake up finish charging to 100%. You'd only know this if you are regularly waking up in the middle of the night and checking the charge level to begin with.

And why questions cannot be answered here. Apple does not give you the option to limit the charge level. That's all anyone here on this user to user only forum can tell you.

Honestly, people are over concerned about charging their phones. Batteries - ALL BATTERIES, lose their ability to hold the same charge as they did when new. It is an inevitable process that your Battery Health will decline. It can't be stopped, so why obsess over something you cannot change? Just use and enjoy the phone and when the day arrives that your Battery Health has declined to 80% as it surely will, pay Apple to replace the battery, if you haven't already replaced the phone by that point anyway.

You can however, provide a feature suggestion to Apple (though I'd hold little hope of it ever being implemented) via this link --> Feedback - iPhone - Apple

This function doesn't work, if I check the battery charge profile, it always just charges to max immediately even if I plug it in overnight.

Why can't apple just allow us to sepcify the max charge manually??

I'm not aware this is something Apple or anyone else for that matter has quantified. It certainly seems to work well. My iPhone 13 Pro Max, which I've had since launch, is showing a battery health reading of 99% after now 10 months.

battery health function keeps track... Thats why I attached the image... you can see within the first 2hrs or so the battery is already fully charged and is at 100% overnight until I disconnect in the morning! I have had several phones and this feature doesn't appear to work for me. Battery is the limiting factor in liofespan of the device, once the unit is opened, apart from the hassle factor of taking it in, its never the same. I prefer to keep my device until it doesn't cope with new iOS updates. the features upgraded with each new iteration of iPhone are of minimal value add in my usage case.

If you don't keep a regular schedule, or a least a regular charging schedule, the phone may have trouble with optimized charging. That's definitely true. But, we can't judge that from one screenshot.

Also, if you have Apple replace the battery, the phone should be absolutely fine after replacement. I generally keep my phones long enough to have the battery replaced once, often twice. They always run as if they were new afterwords. As I just bought a new MBA, I don't know that I can justify replacing my iPhone 11 Pro this year. So, given that the battery capacity is down to 85%, I'll probably have the battery replaced a second time. In all the years I've had iPhones, the only non-battery issues I've had were one speaker that went bad (under warranty) and one screen that needed to be replaced after I dropped the phone on the sidewalk. And I can hardly blame the second one on anything but my own clumsiness.

Not really. My battery charge graph doesn't show the hold to 80% in the middle of the night. But it is indeed holding to 80% in the middle of the night, then charging up to 100% when I get up in the morning. I know this because I have actually been in situations when I needed to check my phone for something in the middle of the night and it's at 80%. When I get up, it's at 100%.

What you don't tell us is when you typically charge your iPhone? Do you put it on the charger when you go to bed at night and leave it on the charger until you awaken in the morning? If you don't, you should.

You are truly getting all riled up over nothing. Charge your phone at night while you sleep with Optimized Charging turned on. Be consistent with battery charging. Worry about your battery when the battery health dips to 80%. Anything else is honestly a waste of your time and energy.