Purrkins, Must share according to you how long to hike in Narrows for the first timer? I am sure melk love to read more from your side. Must share your experience as well.
Hike until you want to turn around.
Hiking bottom up you cannot go further than Big Springs without a permit, this is a 8.5 mile out and back hike. It's SLOW going as you are walking in the river over slippery rocks which is kind like walking on greased bowling balls in the dark. This is about an 8-10 hour hike (with stops to rest and take in the scenery). A bit shorter is Orderville Canyon. You can explore up this canyon for a bit which is narrower and darker than the Narrows proper and nobody is around. This is actually my favorite part of the hike. A sign will tell you when you've gone to your limit as beyond the sign requires technical hiking with rappelling so not for casual / first time hikers. Before Orderville Canyon is an area called Wall Street where the walls are exceptionally narrow and straight up. It's wonderful and many people turn around here. My parents were happy after an hour and never saw any of this.
The Narrows are crowded and full of people. Best to take the earliest bus you can. There are no bathrooms and no place to use the bathroom unless you want to go directly in the river - big no-no - so keep this in mind.
You can rent dry shoes but I've always worn a pair of beat up sneakers and shorts. You'll get muddy and wet and that's part of the fun. If you bring a backpack you'll probably want to rent/purchase a dry bag for your lunch and electronics.
The one must to bring is a walking stick. You can rent these from the outfitters just outside the park entrance. This stick is necessary to keep your balance on the bowling balls and to test the depth of your next step. There are some big drops.
Go slow. Look up. And prepare for lots of people. This one is about the journey, not the destination. The earlier you go the better, the further you go, the less people. Before heading out ask the ranger if they anticipate any storms in the area. While the Narrows might be sunny, there might be a storm 15 miles away. All that water eventually flows into the Narrows and can cause flash flooding.
It's an amazing experience. In my top 5 of hikes and once your hooked you might want to try a top down hike which is a multi-day adventure and a trip of a lifetime.