The bubbles start to appear anywhere from 2 days up to a week. In summer, mine started on the second day. It might take around 3 - 5 days now that it is getting chillier. Sally recommends in her book to throw away the whole thing after 7 days if it does not start to bubble. Actually, this is what I had to do with my first batch as in the attempt to avoid the much dreaded white sugar, I used brown sugar and also peeled the ginger. I guess we all learn from our mistakes.
Whenever you have the bubbles, you can make your ginger beer. However, if you are not yet ready, e.g., do not have ginger or lemons or rapadura that you ordered online a week ago and which have still not arrived (as it was in my case), then just keep feeding the bug by adding the same amounts of sugar and ginger every day or every other day.
The amount of ginger root depends on how strong you want it to be. I would recommend starting with the smallest amount if you are not sure. When it has cooled, strain the ginger, add juice from 2 lemons, and the strained ginger bug. If you want to make the next butch straight away, reserve a few tablespoons of the active bug as a starter and replenish it with additional water, ginger root, and sugar. Adding the starter speeds up the process. Add enough water to make 1 gallon (4 liters). Mix everything well and bottle up. I use Ikea bottles with stoppers. They are just over 1 liter, pretty cheap, and fit perfectly into my fridge door. Leave bottles to ferment for about 2 weeks in a warm place.