The park appears to be open 24/7 Depending on what you want to do will determine how much time you want to spend here. I have been here 2 days and could stay 2 more!
I dont remember hours open exactly but it seemed pretty early and til at least dusk. Its chilly there now so dress in layers, at least warmly and plan a few hours so you can see different views and visit their store.
I would plan at least 3 to 4 hours
For an updated list of what is open and what is closed, visit this page: https://www.nps.gov/grca/planyourvisit/grand-canyon-national-park-public-health-update.htm
7am to 5pm would be the time for viewing when it's not dark. If you are just going to stay on the rim and would like to try to avoid crowds, I would plan to be there from 7 or 8 am to 10am or so. It can get crowded afterwards.
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