The word "loop" here has multiple meanings. Only one is what you mean and the other person responding to you has understood another.
The first is the DDT control loop, what a human driver does. Waymo's remote assistants aren't involved in that. The computer always has responsibility for the safety of the vehicle and decisionmaking while operating, which is why Waymo's humans are remote assistants and not remote drivers. Their safety drivers do participate in the DDT loop, hence the name.
But there's also another "loop" of human involvement. Sometimes the vehicle doesn't understand the scene and asks humans for advice about the appropriate action to take. It's vaguely similar to captchas. The human will usually confirm the computer's proposed actions, but they can also suggest different actions. The computer the advice as a prior to continue operating instead of giving up the DDT responsibility. There's very likely a closely monitored SLA between a few seconds to a few minutes on how long it takes humans to start looking at the scene.
If something causes the computer to believe the advice isn't safe, it will ignore it. There have been cases where Waymos have erroneously detected collisions and remote assistants were unable to override that decisionmaking. When that happens, a vehicle recovery team is physically sent out to the location. The SLA here is likely between tens of minutes and a couple hours.