Batteries that are subjected to fewer deep charge/discharge cycles and aren't exposed to a lot of heat will obviously last significantly longer.
Indeed, but I am the worst for maintaining batteries. I often keep my laptops plugged in at 100%, and then I'll use them until 0 or near 0. Or what I actually do is I just charge them to full, sometimes from say 90%. Sometimes from 50%. Or sometimes from 0. IOW, I just use the laptops how I want to use them (whether it'd be for 50% of the charge or 100% of the charge), and then I plug it in and leave it plugged-in until I use it next. I'm told this is not the way to get the best life out of your batteries.
Despite this, I'm still on the original battery for my 2009 MacBook Pro, so that makes it 6 years old now. The battery life isn't the greatest, but that's mainly because battery life in general from that era wasn't the greatest in the first place. However, I suspect I have 2/3rds of the original battery life, although I'm guessing. It's probably not 3/4, but it's definitely way more than half.
I have seen the expected lifespan of lithium batteries listed as 3-5 years several times when trying to find an answer to this question. Personally, I have had some last longer than five years and some completely refuse to hold any charge whatsoever in less than three years, so I am guessing there are extreme cases either way. I imagine there is a lot of variability due to different stress levels on the battery as well. I'm sure a battery pack subjected to a Qi charger every day will not last as long as a pack that never gets charged wirelessly simply due to heat. Maybe a good rule of thumb is to expect at least three years out of normal usage but don't be bummed if that's all you get.
I think it depends on the software maintaining it. Note my comment above about my 2009 MacBook Pro. This is in stark contrast from my iBooks from over a decade ago. Those things used to go through batteries like mad. It was much worse with non-OEM batteries too. I must have had about 5 iBook batteries in my closet from various iBooks I've owned.
BTW, my hybrid car uses a Lithium ion battery. It's a bit of an unusual case, because the dealership used it as a car to transport customers around the city, but almost never charged it... in 2.5 years before I bought it. I was looking through the charging stats, and out of the 29000 km driven over 2.5 years, only about 700 were in electric vehicle mode. All the rest were in hybrid mode, which effectively meant the extra battery for the plug-in feature of the car was always empty.
Except it wasn't, because presumably that's now how Toyota has designed the battery charging software. I've read that when you "fully" discharge the car, you never fully discharge the battery. I believe there is always at least about 20% juice in the battery. Similarly, when you "fully" charge the car, you never fully charge it, and it's probably below 80% charged. I also wonder if they're rotating the cells charged to spread out the work. All I can say is that although the dealer basically never plugged in the car, the range I get with it as expected with a new one, meaning the battery hasn't seen any significant life degradation over 2.5 years. Or if there is degradation, it's less than 5% judging by what I read online.
Using the claims by some in this thread, that Lithium ion battery should be on its last legs, ready to die completely any time now.
I know a car isn't a laptop, and car analogies never are perfect, but I suspect Apple and other manufacturers are employing a similar method (albeit not anywhere near as drastic) to help extend the lives of their batteries.