A commonly asked question is how long does the rechargeable battery in a G-Shock last, and the usual answer is that no one really knows for sure. Here is what happened to one of mine, which probably doesn't really say a lot about the normal life expectancy of these batteries but I thought I'd share. I bought a used GW-6900 from the bay a few months ago, and it has a J date code so it was produced in 2010, which obviously means the battery has been in service for 5+ years. Of course I don't know the history of this watch prior to me getting it, but I'm going to assume this is the original battery.
Since I've had it the battery level has remained on high until a couple weeks ago I got it out to wear and noticed it was on mid. Since then I have put it outside on sunny days several times, and also on cloudy days as well. I can estimate conservatively that it has been out in full sunshine at least 25 hours and at least that many on cloudy days. The watch has not moved off of mid during all of that time.
I ordered a new battery and put it in the watch today. It was on "chg" and flashing so I set it in the sun and in a matter of a few minutes it went to high. I don't know how much these batteries are charged at the factory, or how long the watch has been setting around waiting to be put to work, so I'm going to let it set out in the sun for awhile just to top it off.
I suppose there could be a problem with the charging system in the watch, and the battery that was in it might still be good but the watch isn't properly charging it. I won't know that for awhile until the new one drops to mid and needs charging, which could be a long time.
So, this battery "seems" to have lost its ability to hold a full charge after 5+ years of service, but I also realize that there are a couple of unknown variables that might have affected the normal life of the battery.