Try 500ish years really. Knights as advanced as those represented by the Empire and Bretonnia would have been around in the 14/15th centuries....
Does Empire fluff say how long they've had their knights?
In the Western world we had the motivation of free market capitalism and regular conflict, and the philosophical belief that progress was possible, and that it was a good thing. Technology advanced in an arms race between making armour that would stop a weapon, then making weapons that would pierce that armour. I read somewhere that 17th Century Spanish plate armour would stop a musket ball while only weighing about 70 pounds. I suppose that would explain why our knights have armour as good as they do.
I'd guess that the ongoing war with Chaos would provide the motivation to develop technology, and magic, but perhaps there isn't the philosophical belief that progress is necessary (well that could be the fault of the games designers who don't want to give Empire too much good stuff). Each edition we get even more ramshackle equipment that is more likely to hurt us than our opponents.
What are we? Orcs?