Based on my experience and what I have learned, I hope I might be able to help you a little bit in understanding yeast and soda making. Now I have been making root beer for the last few weeks but I have been making A LOT of it and doing a lot of experiemnting. Based on my findings, more yest=faster carbonation. When I put my root beer into my 1 gallon jugs, I will use only 1/8tsp of yeast. Then I let it sit for 2-3 days room temp, then into the fridge for 2-3 days unless im feeling realy antsy on trying a batch then it might be 1 day.
I have found using 1/4tsp yeast makes the batch carbonate to much to quick and when opening, the bubbles/foam,etc become uncontrollable. If I am bottling I may use an estimated of 1/16tsp of yeast per 16oz bottle. I dont really measure, I just kind of sprinkle a bit into each bottle. The yeast I use is Champagne yeast. Out of the few yeasts I have used I have found this one (Star brand) to be the cleanest tasting and not make my creations taste like beer. I might add I hate beer and the flavor.
I have also been told, and it seems to be true, that champagne yeast doesnt go completely dormant when in the fridge, it just slows down a bit. This is great as it would be possible then to have pop that never goes flat...lol The bubbles created from the champagne yeast are great I think. So here are some points that you could try and play with and again this is based on my experience.
1. If botteled use a very small amount of yeast such as 1/16tsp or less to start with. I have even heard people using 7 grains of yest/bottle. Of course this means, as mentioned above, that less yeast would = longer time to carbonate.
2. If going into a jug I would start at about 1/8tsp/gallon. I have only been using 1 gallon jugs and this means about 2-3 days before putting into the fridge. I sometimes have enough to even fill a 2 liter bottle (if I make a 2 gallon batch) so that I can squeeze the 2 liter bottle to see how much CO2 is in it. If the plastic is really tight and firm then I throw it all in the fridge.
3. It seems that when using an Ale Pale to bottle my brew, and mixing the yeast in the pale that some bottles seem to get more carbonated than others. I should have seen this coming but I thought I would try anyway. This is why I do each bottle individually now when adding yeast.
Well I hope these points help and get you started in making some soda. Please keep us all posted and let us know how it goes.