The old Templar in Denerim didn't stop taking it. (The warden can ask why he doesn't just stop taking it, to which the other templar says 'he wouldn't be templar any more) It was just the eventual outcome after so many years of taking it.
Cullen had already been off the stuff for months before the game started, given the game likely takes around a year or several months at least, he was probably over the hard part already.
There was a second Templar, a bann's brother who was locked up and denied lyrium by Howe who was also in bad shape. But it wasn't by his choice and he had no support.
I think from things the devs have said that DAI takes at least a year (which seems short to me) to as much as 3, though now I wonder if that was projecting ahead for Trespasser. But by the defeat of Cory, he'd been off it for about a year and a half.
Yes... "good"...
"Mages doesn't deserve be treated like people, they aren't like you or me. They are weapons"
Please, this is a polite thread, where people are civil to each other and supportive about one of their favorite characters. If you just want to throw mud, please don't.