30 minutes. You can't go past the sidewalk
Like others said it depends where you start and how long you want to do it. If you are not into heavy hiking or you have kids you can just go to the narrows "end" and go up as far as you want and then just back out. It is still a great experience. Just check with the park before you go since water levels or thunderstorms upstream frequently close the trails.
If you are talking about "angel landing", then a round-trip trail takes 4-5 hours. And be sure to lay time to rest.
Trail not open
It depends on where you start. I would look up nps.gov/zion that is the web page for Zion and give up to date information about every 30 minutes. Right now the narrows is closed due to high water levels from rain and snow melt.
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