I know that sometimes you don't see the starter activity and it still fermented out. I think it goes so quick that sometimes we miss it. Is it still cloudy, or is the yeast sitting at the bottom in a nice little cake? If it's flocculated, it's probably done. 24 hours should be all you need, but I try to do it 48 hours in advance.
The foil is fine, since you wanted oxygen in there.
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