Time in Zion NP?
A ride straight thru the park will only take an hour or so depending on traffic. (Orderville to Hurricane is only 52 miles) This time of year it should be reletively light.
But....I can't imagine anyone not wanting to stop numerous times for photo ops or just plain gawking at the beauty there.
So, how long it will take is an individual thing. The least amount of time I can remember is 2 hours and that was in November (brrr..).
Where to stop? To many good places to mention, just 'be ready' to stop around every corner. Have fun!
As for route from Vegas to Zion...you are limited but;
I like to ride thru Moapa Valley, so my route would be Lake Mead Blvd (exit 45) to Northshore rd to Moapa then I-15 to St George, . A great side trip would be thru Valley of Fire SP and back to Northshore road, then north or;
Or just ride thru Valley of Fire to I-15 and north to Zion NP.
Be sure to enjoy the Virgen River part of I-15... just before you enter Utah.
From St George you can take I-15 to exit 16 to Hurricane and then Zion, but I think a better ride is staying on I-15 until exit 27 and ride thru Toquerville to Zion.
Time for side trips? On the way to Zion as you ride thru the city of Virgin watch for the Kolob Reservior road on the north side of hwy 9. You can follow that to the reservior and return, or continue to follow it (you will ride on 18 miles of mostly nice dirt road) to hwy 14 (Cedar Canyon Road) . Follow 14 east to U.S. 89, then south back to the other side of Zion (about 60 miles) this really is a nice loop instead of riding thru Zion and back the same way.
Don't forget your hankie because you will be drooling!!