- The Only thing we know for sure is that, assuming no atypical outside interference, a Stargate will only remain open for the maximum 38 minutes if you are sending something through it, such as a radio signal.
- There is nothing that has officially stated there is a sensor near a Stargate that detects if someone is near it or not.
- The only reason that gates stay open and or close as they do is for dramatic effect in the show.
- It is possible the gate was in "shut down mode" before Michael ran through and that there was no way to "stop" the gate from its "time out."
So first, I think it is important to point out the obvious fact that a Stargate can only remain open for the full 38 minutes when something is going through it like a radio signal or incomplete matter stream (Jack holding the gate open during the episode where the NID are stealing stuff). This of course is dependent upon not having a black hole or something else affecting the normal function.
Second, there has never been anything official that said a gate has a motion sensor to nearby objects. And if there were I have a feeling there would have been defensive measures in place to keep a connection from occurring if something is in the "kwoosh" on the other side to keep it from being destroyed.
The only thing we know for sure is that a gate has protocols in place that it can extrapolate if something is actually trying to get through based on the pressure it exerts; from the underwater gate episode. (What is interesting about that however is that if dirt and rock can keep a connection from forming with a buried gate, why does water not do the same thing?)
Without going through and timing each gate occurrence in every single episode for how long it stays open without anything passing through it we do not have information on how long a gate will "wait" for something to happen. That has never been identified.
It is possible that the reason Ronan did not make it through is that there is a sort of "shut down mode" in the protocols. Let's say that a gate has such a mode and nothing happened so it enacted the shut down, which takes a few seconds to occur, and in between Michael goes through. Hence the gate was in shut down as he went through and Ronan did not make it...
But that is making assumptions based on information we do not know.