Been doing that forever for warm weather transplants. I use gallon size with the top open, mainly for wind protection. Provides very little help against cold. I would think when you begin to warm up they can be removed in a week or so.
My brother lives in the Highlands. I go down the second week of April and set out many pepper and mater plants. These are protected until the plants start spilling out over the top of the plastic, usually in about 2 weeks.
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