Well, for R/C use, the `rule of thumb' is 80% or 90%. But R/C is a bit more critical than most uses -- if the battery goes dead mid-flight, your plane crashes, and you may not have much warning that this is happening. If your headlight batteries die, you usually have it gradually getting dimmer and dimmer before it goes out and that's very obvious. And if you're properly equipped, you've got a spare set of batteries with you -- or a spare light. So there's no need to be that careful about your batteries -- even 50% life left is OK as long as that gives you enough juice for a typical ride without any drama. And yes, batteries that have lost a significant amount of capacity are more likely to go suddenly dead with no warning, but that's rare and immediately obvious, and usually not a crisis. (Assuming you have more batteries or lights, of course.)

(Outside of R/C use, I'm not sure there really is a rule of thumb. People replace batteries when they stop working. Often they ditch the entire device!)

Ultimately, replace them when they don't last long enough for you anymore, with `long enough' being something that you decide based on your needs.

Do be a little more careful about rear-lights, however -- especially if you only have one. It's easy to miss them getting dim and going out.

But yes, don't use those 15 minute chargers -- a full charge (from empty to full) should take at least an hour, and even that's on the fast side for AA sized NiMH cells. (It's usually fine for sub-C sized cells, however.) It's OK if the battery gets warm, but not if it gets hot.

Also, get a charger that turns itself off. The good ones detect the `peak' when the battery is fully charged and turn off automatically. The cheap ones just have a timer and turn off after X hours -- not as good, but still better than the ones that have no timer at all. Leaving NiCd/NiMH cells on a slow charger that never turns off wears them out over time. (Putting them on a fast charger that never turns off ruins them quickly.)

LiPo and Li-ion chargers all turn off when the battery is full, so it's not a concern there. (If they didn't turn off, the battery would be ruined the first time it happened and perhaps catch fire.)