I have done the bottom-up to Orderville and back about 5 times. Most recent trip was over 5 years ago, however, so my timing is a bit hazy.

One thing about the narrows is that timing is very much based upon the level of flow of the river. I have gone when it has been a trickle and seemed like to Orderville it only took 1-2 hours. You could walk in the river course and on sand bars. Other times where the flow is higher, you are fighting each crossing and it can take much longer.

I like to explore up Orderville canyon a bit and you can go up in the narrows a bit past the confluence. You can easily get to the heart of the narrows from the bottom and that way you don't need an overnight permit or a car shuttle from the ranch.

Overall, I would plan on 3/4 of a day and do some exploring. If you get done earlier, you can always add one of the shorter hikes, like Emerald Pools, the Grotto, or a hike up to Echo.

Have fun. I love Zion.