embarassed to say
When I did it regularly it lasted 3 weeks.
I'm embarassed to admit that I have not been regular at refilling it at all! The Hagen unit has a series of small paper tags that I add that says what the recipe was that I used (whether bread yeast or beer or champagne, volume, baking soda volume - I varied these a lot over the past years) and I note when I refilled it, if I washed the bottle.
When I do check it now I often see that it was 2 or 3 months since I last refilled but that is because I just let it die completely. Sometimes it would still be going, and the tag would say 6 weeks or 2 or 3 months and I have to think that I just neglected to change the tag then, "Naw, that can't be right!" But, when I was obsessive about it, it would last 2 1/2 to 3 weeks. And I'd change it when it got to only 3 bubbles on the lader at one time, since that was pretty close to how many bubbles there were per minute. I'd start at 6 or 10 bpm and decline to 3 then I'd change it.
I am overextended in fish tanks now for the 2 discus tanks take up most of my time devoted to getting my hands wet now with their 3x/week water changes. The little 10 gallon was getting ferts and new CO2 only if it looks like the vals are dying back. The broken line tetras in there were always hiding in the thick plants and I often forgot to feed them. I'd tend it when the Whisper mini got so plugged up it overflowed back by the input line or if the ambulia or water lily took over the tank. So it was terribly neglected and I just recently broke it down, sold most of the plants at auction and sold the tank and Hagen and lights and flourite locally. I still have one Hagen not in use now.
My advice is to add the sugar (never overfill the sugar), then 1 teaspoon baking soda, then skin temp or a bit warmer water, then 1/8 to 1/4 teaspoon bread yeast. And watch until the yeast falls then rises again in the water. Seal it up, give an extra 1/4 turn. Put the ladder as high as possible in the tank in an area of good flow.
If you do experiment, just keep good notes of what you did and how it worked. I've used beer yeast, with yeast nutrients from the beer brew store, I've used champagne yeast. I found the bread yeast plus baking soda worked dependably. Sometimes I'd not rinse out the jar, just pour it out and refill, but I never got a true opinion on that, whether it was good or bad, seemed to go both ways either worked great or bad early. Just have fun with it. Eventually you'll see what you think is best.
Member AGA, HAS, NASH
110G, 10 adult discus, barebottom with plants on wood
58G, 3 adult discus, barebottom with plants on wood
59G thickly planted tanks, rummey nose tetras, kuhlies
300G goldfish pond outside