In Stargate SG-1 when you fire a Zat the discharge increments as the following: 1 hit for stun, 2 is a kill and 3 disintegrates. But how long would you have to wait to hit again and be okay? Are they cumulative? If I was hit once now, in six months time if I was hit again would I be stunned or killed?
Further, when a character who has been knocked unconscious by this effect within the last 20 minutes is successfully attacked a second time with a zat, the attack is automatically considered a coup de grace action.
There is no number I know of in the show, but I did just watch the episode "1969" where Jack Zats 2 cases of stuff that disintegrates and it requires 3 shots. This implies that the requirement is something to do with the energy where the increasing amount of energy is enough to do the stated damage. In other words, it is the energy of the Zat combining to intensify and have the effect of the said level of shot. This then implies that the cause of the first and second shots effects is lower levels of matter being ripped apart. So...
1 shot, is delivering enough energy that the matter being ripped apart is just painful, but healable.
2 shots, is delivering enough energy that the matter is being ripped apart to cause large amount of damage, probably to organs.
There is a difference between having your skin cells, nerves, or even blood being ripped apart, which would be happening with the first shot, and having internal organs and neurons ripped apart.
The third shot then is just an increase of that ripping apart...
So how long after the first shot can you be shot again? The answer to this then is that it should be "safe" to be shot after the energy dissipates which should mean when the electrical stuff stops appearing, but because we see that you can be shot after that disappears it's not really possible to tell. The closest we can come to getting an answer is if you have scene that continues on or we know the time between the scenes and you count the time that passes, between the first and second shot and assume any longer will not result in death.
Also, assuming all this is correct. you wouldn't be "safe" to be hit again even in that time span, because it takes longer for most of the things that would be affected by that first hit to be regenerated, meaning that if you knew the exact amount of time for the energy to dissipate you still wouldn't want to be hit again for a while, because you'd be sustaining damage which would heal at a variable amount per species and person. For example, shooting Jack after the energy disperses could kill him anyways due to him not recovering enough yet, where as Teal'c has stronger regeneration from the Symbiote and thus might be perfectly fine with that second hit of 1 shot.
Regardless, 3 shots would disintegrate anything though so long as the energy hasn't been dispersed and nothing is holding it together beyond that... like say, you can't use it on a planet or star because it's already mostly just matter that is clumped together in one place and held together via gravity. Disintegrating such things would just result in them remaining the same as they were previously... Though you could do it to any surface thing, or ship...
The best way to get reference for this is in the episode Sam brings the cadet to the possible alpha site and everyone is attacked by the (for lack of better name) energy sprites and they shot jack to disrupt his natural electrical energy. I can't remember but I believe Sam said it would be about 7 minutes before the energy returned to normal. If that is so then we can assume that when the body's electrical charge returns to normal you can probably be shot again and it will only stun.
The time varies. I was going to write something very similar to what I ended up finding on a SG Wiki:
The effect of the weapon on the subject varies considerably. In some cases he is left unconscious for a considerable length of time, for example when the disguised alien Tyler zatted Daniel at very close range and Daniel had to be awakened by Carter sometime later (5.04 "The Fifth Man"). In other cases the person is disabled for only a few moments, for example when Anubis used Carter's body to zat Daniel in an SGC corridor. He was in pain but appeared never to lose consciousness, and he was able to stand a short time later (8.03 "Lockdown").
Also unknown is the maximum length of time that can pass between shots for the second shot to be fatal or the third shot to disintegrate the subject. Clearly if the second shot is within a few seconds, a typical human subject dies. If the subject has recovered from the first shot, it is assumed that the second shot is not fatal. Humans in extraordinary circumstances may survive a rapid second shot, as Carter did when she was possessed by an alien consciousness and O'Neill was forced to zat her twice (4.20 "Entity").
Throughout the show, we don't see a conscious, recently Zat'd individual ever just die. Whatever disrupts people's energy that leads to unconsciousness seems to dissipate when the regain consciousness.
Since we see throughout the series different individuals being affected by the first blast longer or shorter depending on the character and plot need, it's probably safe to say that there's no standard timeframe for when the second blast doesn't kill. Instead, if they're up and going after the first blast, their counter is probably reset.