I got my Clip used from someone who owned it for about a year and a half before I got it so it's been 3+ years since I got it and I've used it heavily.
On a full charge, it'll give me a solid 7+ hours before the battery takes a nose dive (once it gets down to 25% capacity, it drains rather quickly), and a little less (maybe 5+) if I'm listening really loud or with lots of bass.
The battery has a 400mAh and Sansa claims a fresh unit has a 14-hour capacity, although I'm sure that's a conservative estimate.
Point is, if your experience is anything like mine, (luck of the draw, perhaps?) it's unlikely you'll be needing to replace the battery unless you're looking to hold on to it for more than 5 years, at which point, I would expect battery capacity to go south no matter what it's made of.
Awesome! I've had to re-solder the battery wires on my Clip twice just because it bangs around in my pocket so much, but never replaced it.
Good to know it is possible, but I was pretty paranoid every time I had to crack the case open; plastic snap brackets just aren't as robust as proper fasteners.
The post by "ncuxanam" in your second link has a link to the proper replacement, BTW.