We do not currently know which dungeons will be used in Legion Timewalking, but it has been announced that there will also be Mythic+ versions of them during the event. The first Legion Timewalking event will last for 2 weeks, to celebrate its introduction with subsequent events lasting a week.
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On Blizzard's Patch 9.1.5 Content Update blog post, Blizzard announced some more information on Patch 9.1.5, including the introduction of the Legion Timewalking event!