Answer: The answer really depends on what seeds you're starting, but a general rule of thumb is to remove the cover as soon as the seeds sprout. Seedlings in overly moist conditions can develop fungal diseases. The covers are designed to help hold in moisture and to prevent cold drafts. Once the seedlings are up and growing they'll need good air circulation. If the seeds require extra protection, you can remove the cover once a day to allow excess moisture to evaporate, or you can prop it up to allow fresh air to enter. But, with most flower and vegetable seedlings, you can completely remove the cover once the seedlings have emerged.
Hope this clarifies things for you!
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