I was reading this question, and realizing that I don't have a good handle on this question, namely how long can a ZPM power the chair for? Thanks!
There's not really much data on this, but we do know that powering the chair would take about 80% of the US's power generation capability (in an alternate universe).
Therefore, powering the chair is something that is feasable (power-wise) for the United States to do.
All of the estimates I've seen for creating energy shields, opening wormholes, etc are far beyond our current capabilities, yet the ZPM can do it for extended periods.
Therefore, I expect that a fully charged ZPM could power the ancient defense chair virtually indefinitely - certainly for a matter of years, if not decades (or even centuries).
There is never a clear illustration of exactly how long it would be, but there are some points of evidence. Let's discuss those first, then see what they mean.
- The chair in Antartica was powered by a ZPM found on a distant planet. The power seemed to drain very quickly after it was in use, only service to allow for one trip to Atlantis after the ZPM was deleted.
- The weapons used in the puddle jumpers seem similar to the ones used by the chair, albeit less powerful. Still, as powering the puddle jumpers doesn't seem to have any huge power problems most of the time, it seems the power requirements aren't all that significant.
These two pieces of information are on vastly different scales, which seem to contradict each other. Still, I'll give it decent odds that the ZPM was already pretty depleted after being returned to Earth. Also, there were thousands of shots coming from Antarctica, which probably drained the system even quicker.
All in all, I would estimate the chair has a very significant drain on power. It seems likely the weapons emanating are a significant chunk of nearly pure energy, which would lead to an enormous draw of power.
It is also possible that the Chair was powered by some kind of capacitors, essentially increasing the instantaneous burst.
All in all, I would guess the amount of time that the chair could continually fire to be quite limited, I would be surprised if it lasted for a day or two of continual firing, without anything else drawing power at all.