A little more than an hour. Long enough to pull in a swarm. A 2 litter would last longer but I just get the flavored sparkling water. I open it just to the point I can begin to hear a sound.

For a longer lasting and potentially cheaper source of CO2 just add sugar and yeast to water.

I use a CO2 tank to re-carbonate my soda stream cannisters, I imagine at a slow leak rate my 10lb tank would last a long time and it's only $20 I think to refill it.

I've found a number of systems on Amazon intended for adding CO2 to aquariums but never got around to buying any of those.

Anyone tried using those for mosquitoes?

Just curious: do you notice a difference in the population over time, or is it one of those things that mostly just works while it's running?

It could be my imagination but it seems like there are less of them overall. For sure when I use the disposable fly traps there are less flies overall. I can't stand seeing horses eyes covered in flies. If I were a horse that would drive me insane.

I think in addition - depending on where you live - controlling surface water would be a factor. Don't leave water to go stale in puddles or containers, and if you have something like a pond, either treat the water with something or get plenty of mosquito-egg-eating fish.

What fishes eat the mosquito eggs?

you'd be hard pressed to find a fish that wouldn't like a tasty larvae