This information was orginial posted by dinnerstick credit where credit is due.
This site refers to GBP as SKG or Sugar Keifer Grains
Regarding Growth-Rate of SKG SKG that I have cultured over the years, including many individuals who culture SKG, have observed a fluctuation regarding growth rate of SKG. I have observed growth increase to vary between 7% to 220% increase by weight at 48 hours. However, if the sugar solution with other added ingredients, and more importantly, with the addition of a very small amount of sodium bicarbonate and eggshell, if the solution ingredients are kept constant, including the amount SKG used and the fermentation time is kept to 2 days per each batch, then growth increase can be expected to remain reasonably constant between 90% to 120% per each batch of water kefir. On the other hand, milk kefir-grains increase at a reasonably constant rate when cultured in fresh milk. Many individuals including myself, have found a great potential for SKG to cease propagation, and remain non-propagable, when cultured in water with low mineral content. More so with activated charcoal filtered water. The outcome is mostly observed as a slow deterioration in growth over time, which at a point, becomes irreversible. At this point, SKG acquire a light brown film over the surface of each grain, and the grains lose the typical transparency of good growing [propagable] grains.
Although there are many factors to consider, the important one being the type of sugar and water used, duration of fermentation including sugar/water percentage. SKG do not appear to grow well, and in most cases growth ceases altogether over time, when cultured in a sugar solution prepared with purer forms of water, such as distilled, demineralised or activated carbon filtered water [Brita filtered water e.g.] etc. Leaving SKG in the same sugar-solution for longer than 3 days over many batches and in warmer conditions, has an adverse effect regarding growth-factor of SKG. Lack of essential nutrients and energy source due to over fermentation, is a cause for problems, if starvation happens over an excessive number of proceeding batches.