Not long. Built it last fall. But white oak will stand up to moisture for a LONG time.....reason it was used for water barrels and cabin shingles. It has a closed cell structure, known as tyloses.
I got the idea from greenhouse tomato growers down in Grainger County, TN. Since my hoophouse is only 20' wide, I used no center support. I do have 3 pcs of 1/4" cable tying across the hoops in 3 places to "draw" them inward so they don't push my kneewalls outward.
The tomato growers use much wider houses.....about 40' wide. They run a series of 4x4 posts down the inside, dividing the house in roughly four 10' wide 'bays' across the width. The posts are topped with a beam made up of couple 2x6's running the length of the house. Top of the beam was something like 9-10' for the two left/right side rows, and about 12-13' for the center row.
Then from their outside walls ( which are 6' tall ), they took 1x6 treated boards, laid them on flat side (6 " width) from outside wall to the 1st beam, arching on over the top beam, back down to the other side beam, and to the other outside wall. They used 3 laminations, breaking the joints, of course, at different places to form a laminated arch. I don't think they glued them ( I did glue mine....Titebond 3 waterproof glue ), just nails or screws.
Worked real well. Since you plan to support your inside anyway, I would go all wood and just skip the tubing.