Sam Darnold suffered a soft-tissue glute injury in Week 1 (Sep 16) and has already been ruled out for Week 2 vs. Arizona. The Seahawks have no plans to place him on IR, which typically signals the team expects him back within about 4 weeks — but he could return sooner (from the Sam Darnold page).
On Mayo Media Network, they noted Darnold is out "multiple weeks" and pointed out the Seahawks have some manageable matchups coming up against the Giants and Commanders, which could buy him extra recovery time before a return .
So the rough expectation is return by Week 5 or 6 (early October) — but soft-tissue injuries are tricky, so keep an eye on Seahawks practice reports. Drew Lock is filling in for now.