And there is a lot of middle ground in the game between mobilizing with low war support and mobilizing with high war support. 4-5 years is only applying to countries with low war support.
I know about war-support which is why my very next line was "It should maybe take a few months to a year max depending on war support."
War support gives +3% mobilization speed at 100% war support and -50% at 0% war support. I tested it as France when I had 92% war support, I was getting 0.009% per day. That means 0.27% per month or around 3.2% per year.
Assuming you start at "limited conscription" (and democracies cant go beyond that until the war starts), it would take 5+ years to get to all adults serve with MAX war support. And this is where this conversation started.