I'd say a few months to become some kind of lesser lord to a Demon Prince, then a year or so to overthrow one and steal his position...
And then some time to consolidate himself and gather minions.. Then overthrow the local King..
I'd say about two or so years to become the Kind of a demi-hell dimension. Then a few more years to root out any possible threats..
Four, maybe five years is my conservative estimate for a local area.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather