I believe the clamping time for Titebond III is supposed to be 1 hour although some claim that 30 minutes is sufficient. I normally allow 2 hours; it can be worked then if force is kept to a minimum, although full strength requires overnight at least. Titebond likes plenty of pressure, more than one would need for epoxy.

I have only once had a scarf joint break, it was on a gunnel and the joint was under considerable tension; Titebond is not at its best with high shear forces and the joint did not have enough area.

BTW the gluing jig doesn't clamp the actual joint - the 2 lever clamps are just to hold the 2 strips in place while the clamps are put over the joint. The clamps stay on until the glue has set while the jig can be moved on to the next joint after a minute or so. Once the Titebond has soaked into the grain it ceases to be slippery.