It depends on what you are making and the ingredients. Pectic enzyme doesn't work well in the presence of sulfite, and yeast inhibits the action as well, so the usual procedure for country wines is to mix up the must with campden, wait 12 hours and add the pectic enzyme, and then wait 12 more hours and add the yeast. So, yes, 24 hours after mixing.
But if you're not using campden tablets (sulfite), then you should add the yeast right away so that wild yeasts and bacteria dont' take hold.
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