My wife went through 2 Istick50's in about a year using one while the other charged. They are rather cheap but still its a waste to toss a perfectly working board cause the batts crapped out. They were the first and last internal battery hitter I will ever buy. I took one apart and one day going to replace the batts since it doesn't look all that hard to do but I have other hitters so I'm in no hurry.
Just to avoid getting poo flung at me..my wife tootle puffs at less then 15 watts.
I had a 30W istick a while back that simply had less and less life, and at the end could barely last a 1/2 hour's worth of vaping.
We followed it with a 50W to replace it, which lasted a good 6 months until we got a few Innokin Coolfire, but by then I started to get mods that were 18650 replaceable ones...
I ended up giving my 50W to someone (with a tank) as a starter kit and as far as I know, she might be still using it (as for the following year, I knew that she was still using it). It's been a while now, but odds are that it's still working.
The Coolfires must be getting close to 3 years old now if now more and are still working fine (I only use them sparingly with one of my RDAs, even though at 40W, it's kinda low).
It really does depend on the battery's quality and usage.
And for your comment about replacing it, I'll say the same that I did already, as long as you're careful with soldering a battery and use a proper one (as per my other comment, that video that someone posted, shows someone putting in a low drain 6A type, which isn't something that I'd recommend). But with a decent 15-20A battery, done right (the soldering) is should be fine.
Note that I've seen a few mods that some people tried to do this and it didn't end well, the mods ended up with a vent, and worst part, one person insisted that it was fine and kept using it, even with part of the mod having melted in part, but since the battery was still outputting some power, he kept using it until it died.
