How long do I have to charge my Nikon D5100's battery?

So my Nikon D5100's battery died so I'm currently charging it. It is taking forever. I mean it says it still charging and it has been 2 DAYS. I'm scared that the lithium (I think) might explode. So can someone tell me what's wrong or how long this will take!

No, the power will cut off before there's any danger of that happening. There's obviously something wrong, so I suggest you consult Nikon.

It should take no longer than five hours. Most of us charge them overnight, just before using the camera.

You should NEVER let your battery die completely and if you happen to do that, it needs to be recharged immediate. Do NOT wait, since this is the one thing that can reduce the life of any Li-ion battery

Did you read the owners manual? The light quits flashing when the battery is fully charged.
Maybe time for a new battery.

If the light on the charger is flickering remove it right away, something is wrong with the battery and you might need a new one. If it hasn't charged by now, it's probably a goner.

Also, the Nikon recommendation is that you run the battery flat until the camera doesn't function anymore and then remove it to either store it or recharge it. That is actually in the D5100 manual. What will damage the battery is to let a fully discharged battery sit in the camera, because the camera will still try to draw charge and over discharge it and damage it. So you should alway remove the batteries of any cameras you won't use for a while.