After a surgical repair of a zygomatic arch fracture 2 weeks ago, how should recovery go as far as mobility of the jaw? When should it improve?

Zygoma Fracture. Zygomatic arch fractures can impinge the lower jaw causing decreased mobility. After surgical repair there is swelling that can hinder jaw motion for a few weeks. Your jaw motion should be improving soon. Discuss this issue with your surgeon.
Arch Fracture. The trismus that you are experiencing needs to be evaluated just as dr. Scuba has indicated. What was broken zmc vs isolated arch only. The approach to your fracture can also play a role - gillies approach vs. Keen approach as well as what was actually done to reduce your fracture and stabilize it (rigid fixation vs. Reducation without fixation for slightly displaced arch only fractures.
ZMC fractures. Sometimes the arch and/or the cheekbone (zmc) aren't easy to elevate enough to allow the lower jawbone's more anterior process to swing free underneath it when opening the mouth. Sometimes that repair needs reevaluation if there is continued restriction--the sooner the better. Make sure to discuss with your fracture surgeon, and that he looks carefully for "mechanical" obstruction to opening.