The short answer is "it depends upon your local conditions." I've seen it take as short as two weeks to as long as several weeks. I live in an area that gets snow for a significant portion of the year, and our springtime weather can be a true rollercoaster. Rarely does it get warm and stay that way long enough to get my plants hardened off in just a couple of weeks. Instead, I haul them in and out as the weather dictates, so hardening off takes a lot longer here than when I lived in the South.Â
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Keep in mind that a plant's age alone does not make it acclimated to the outdoors. After bringing the plants indoors for several months, my 3+ year overwinters must be hardened off just like the new plants.Â
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It would help if you tell us what you've already done to protect them and gradually introduce them to the great outdoors. Also, pictures of the plants would help, so we can understand fully what is going on.