In the "Dormammu, I've Come To Bargain" scene, was the time loop keyed to restart immediately after Strange died or was it simply set to start again after a set period of time, perhaps a few minutes?
I tried to find a full movie script but it doesn't appear to be released or leaked yet, so I would have to guess that the reset was on the moment of his death. If it was a set amount of time then logically all of the times he was trying to bargain could prove difficult and the loop could reset in the middle of the conversation. Conversely when Doctor Strange was killed instantly then Dormammu would be sitting there waiting a few minutes, probably thinking he might have finally ended it and won, before the reset occurred.
From what we see, I believe it was set to loop every time he was killed.
We can infer this based on seeing him survive for varying duration in every iteration. Some times he dies just instantly, some times he does manage to buy some time.