I use jello for the sugar mix, read about it and it seemed to last longer, and this has been my experience - it takes the yeast longer to "burn" through the jello mix. But, it also slows down the bubble rate some.
I use 2 small jello packs, made per box instructions. Add 2 cups sugar with the jello mix. Add 1/2 teaspoon of baking soda before puttint it into the fridge, and shake it slightly (It'll fizz up, it settles back down.)
Once it it set, I put 1 cup of water with 1/4 teaspoon of yeast on top of the jello. I find I can reset the yeast mix by pouring it off the top and putting new yeast on the remaining jello after a couple of weeks, and they last me over a month.