Unfortunately if this was the way things worked, literally everyone would just whip out a pistol and demand DeathRP once they were downed. In a realistic world, one or two bullets would be sufficient to incapacitate a suspect without killing them, however cause this is a game and someone getting two-tapped would be unfair and boring, more shots are required and as such the number of shots required to kill an individual is a fair number higher than what would be required in real-life.
Typically I will approve DeathRP if it is realistic, ie. why is the person evading from me? Do they have a tonne of active charges? or is it a simple citation stop they've turned into a Felony Evasion and Attempted Murder of a Gov. Employee for no reason? In addition, is this the first time I've attempted to detain the same individual for the same charges? I have no interest in consistently having to chase the same person for the same charges every-time for them to whip out a pistol and go all last stand on me for the umpteenth time.
I don't really deny DeathRP based on the length of time that a situation occurs, I typically deny it more for poor RP like I've mentioned above. (Although it is rare for me to be in a position where the complete decision of DeathRP is on me alone.)
If there was a system where DeathRP had consequences similar to jail, then I think the whole scenario would be more fair.