Baking Soda helps get more life out of your DIY CO2 mix. I use it and I get well over 3 weeks of CO2 from a 1.5 ltr bottle.
The baking soda helps the yeast deal with the alcohol, so they dont die off so quickly. Ill add my mixture incase its usefull to anyone, 3 weeks is pretty good im guessing.
2 cups of castor sugar
1/2 a cup of brown sugar (can use castor but I find brown is what the yeast is still feeding on in the 3rd week)
about 3 healthy pinches on yeast, make sure to get 100% active yeast, not brewers yeast or bread mix yeast. They work but not as effectively.
1 pinch of baking soda. not much but it makes a difference.
1 ltr of luke warm water.
Mix the sugar and water out of the bottle to make sure that the sugar has completely disolved. Empty into a bottle, then add the baking soda and yeast. Give it a wee shake but not too much.
I get this bubbling at about 1bps within 6 hours, tempreture depending. I put the bottle on top of my all in one tank and it gets heat off the light, at night without the heat the CO2 production dies down to just about nothing. Because its getting colder I've also wrapped the bottle in an old woolen jumper, any insulation will do though.
HTH