Whenever music is playing the point that you connect the wire to is activated so the light will be on. I'm sure this goes for YouTube as well.
The light will last much longer than your phone, that's for sure. Even if used 8 hours a day, a standard LED will last over 25 years.
Yes, be very careful if you use this mod(especially during installation). The replacement back piece that they give you is made from a much thinner glass than the stock iPhone case(because there needs to be room for the LED panel to sit on top of the battery). I have heard of several people breaking the glass during installation.
My glass is still in great condition, I use a bumper for my 4S. However, I am always especially careful with my phone.
You should try contacting K.O. Gadget to see if you can possibly buy just the glass, but I am sure the glass is the most expensive part anyway so it probably wont be any cheaper.
Does the glow kit work with the iPhone 4??
I've gotten the light to turn on and all...but the light just doesn't shut off. At first, it'd shut off at 15 seconds or so but then turn right back on and repeat. Now, it just stays on, anyone else having this issue? Any help would be appreciated, thanks.