For the vast majority of the quest, the time period is described as 20 years, so I'm assuming that is the proper answer. But in some of the initial and early chats the time period is described as 16 years.
Gweyr - initial meeting
"It was about fifteen, sixteen years ago," Gweyr says. "Four years since we settled Hawkethorne, three since Garth and I finished building the Frost Hound together.
"Sixteen years," Quin muses. "Didn't know the bastard had been doing this for that long.
Gweyr (Rift) - Talk - Living
"Ah, the gods are merciful in their own way, aren't they? The chance to finish the job I should've closed sixteen years ago..."
Gweyr (Rift) - Talk - Garth
"Haha! Ha! Sixteen years, and he's still running to the same old dogs to get stuff built.
Gweyr (Rift) - Talk - Kids
After fifteen, sixteen years out here, I just want to go home to my husband and children, make peace with myself so I can do that."
Garth - Talk - Gweyr (one off)
He could've at least told you and Cait about what happened sixteen years ago, when Tollus first arrived and tried to annex the fledgeling village.
