The thing about fermenting is that even though your culture takes a long time to get to its climax, it isn't very demanding of you. It's like having a very forgiving boyfriend. You don't have to be punctual. Whenever you show up with a little sugar will pretty much always be the right time, as long as you don't wait so long that you create the circumstances for an explosion.
I see them on sale at the supermarket as well. So, if you're not growing any raspberries, it might be a good time to find a good deal on some. But a couple of handfuls of raspberries is all I need to make this very refreshing soda that tastes so delicious over ice