Hi Amy! You'll want to cover it with something to keep it from drying out. Plastic wrap, foil, an upsidedown plate or pan, a wet towel, etc. Plastic wrap is usually the easiest. If you're talking about mixing just the starter, it's until combined. If you're talking about mixing the bread dough, it's until it's bouncy like a marshmallow but there's no need to be aggressive with it, it'll take about 8 to 10 minutes by hand or 5 to 7 in a mixer on speed 2. Call our Baker's Hotline anytime you have a question or consult our Sourdough Guide. We're here to help! 

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