This has to have been addressed or explained at some point. I haven't tried searching for it. I'm just going to ask here.
What determines how long the wormhole stays open during a non-emergency? Specifically, someone activates it, a group of people walk through and the Stargate just happens to shut down after the last person. Why not five seconds later, or thirty?
And why does it stay active if the group takes its time? If person 2 starts walking five seconds after the first, and then the third person fifteen seconds later, why doesn't the Stargate shut down before the last guy gets there?
If there's still something passing through it like a radio signal or a person stops half-way into or out of the event horizon, it will stay open. Jack O'Neill kept it open one time by holding his gun at the exit of the event horizon, but didn't he fully exit from the Stargate and turn back around to stick his gun in? Why did the Stargate accept matter trying to re-enter the exit?
The simple answer is the MST3K mantra: don't think about it too much and the answer is whatever the story requires is how long it stays open. But what's the in-universe explanation?