As long as it’s coagulating within 24-hours it’s active. You could measure grain growth too. If the grains weren’t active, you’d just be left with sour milk after 2-days.
I don’t think the test is a great measure. When I make plain water kefir, I get next to no gas. When I make it with grape juice, it can blow the lid off the jar. The gas only tells you how well the yeasts are growing (C02 and alcohol are by-products). The alcohol volume is unlikely to go above 1%, so it’s perfectly fine to drink. I think 2% is the upper-bound.