How about a CMOS 555 timer? Some aren't meant to run from more than low Vcc, so make sure the one you choose does. Should draw on the order of microamps when the LED is off.
The TS555 is rated to 16V. Just put an appropriate resistor on pin 5 so pins 2 and 6 trigger when the 5V line drops to where you want it to trigger. Built-in hysteresis. It acts as an inverting schmitt trigger.