Glow plug light

[mxpro]

Cold weather is coming, and just wanted to see if someone could tell me why the glow plug light on the dash doesnt stay lit as long as it did when new.
Last year hard starts when below 15 degrees had to use block heater ,but never had to for the first four years. The truck is a 2005 f 350 Innovative did a reflash on ficm , and I use sct tuner with tunes from Eric at Innovative.
Starts and runs great when block heater is used
.
Thanks for any help