Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Watch the bubble rate.
The first day or two, the bubble rate usually increases. Then after that, it holds fairly steady, dropping a bit over time. At some point, it will start dropping rapidly. That's when it's time to change it, but preferably a day or two before. For most recipes, that's a week or two.
Since this is your first batch, check it daily, and count how many days it lasts before the rapid drop. Then, as long as you keep the recipe the same, you can just mark your calendar for when to replace future batches.