I've got some older white LEDs originallyh from Real Goods, that have been running 24/7/365 for around 7 years, and they are definitely showing degradation. These are dimming significantly (I wish I had kept a few unused for "control" samples). They now have a yucky brownish tinge, a couple have failed altogether, but still produce semi-usable "night light" illumination. These are in a series-parallel configuration, three in series with a 100 ohm resistor, powered by a 12VDC wall cube, pretty close to nominal 20 mA current.
I also have some Nichias that have been going over 4 years, they seem to be holding up well. A 36 LED array using some of the white LEDs (manufacturer unknown) from SuperbrightLEDs has been running continuously for 3 years, three of the LEDs are noticeably dimmed down, the rest are still full brightness The are in a an arangement of four groups each in a 3X3 series-parallel with a 100 ohm resistor in series, also feeding from a 12V wart. THis is my bathroom night light.
/ed B in NH