2004 Citroen C8 economy mode active

How long is it lit foras they do stay alight for a short while whilst systems shut down(Power latch)

Agree with above . Happens with other cars also nowadays i.e Fords backlight I think stays lit for 5/10 mins .

I first had the EMA warning when the car was 2 years old. After 3 years of EMA warnings the battery died. Had new 5 year Exide battery fitted by RAC. Now after 18 months I have Economy Mode Active warning. I expect another three and a half years before the battery dies. 5 years is a short life for batteries these days, I would expect 9 to 12 years from experience: 206 10 years, Corsa 9 years, Trajet 12 years! Clearly the Citroen electronics is wearing down the battery prematurely.
Citroen GB say the EMA system cannot be adjusted or turned off.
When the battery is nearing 5 years old expect glitches. Expect bogus particulate filter warning. Expect electric mirrors to not fold. Expect heater fan and air con to not work until have driven for 20 minutes. Expect cruise controls to freeze.