When I first started brewing, I took one after 1 week to ensure there wasn't a stuck fermentation, etc, and one before packaging.
Once I got about 10 batches under my belt, I moved to one just prior to packaging. I have a precision hydrometer and it takes about 12-13 oz beer to fill the tube. I was more confident in the process and didn't want to waste a beer!
Cheers
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