When should a person start physical therapy and mobilize his elbow after elbow dislocation/fracture ?

Within a week: Assuming your fracture-dislocation was fixed surgically (as most are), it is critical to get your elbow moving within a week of surgery. You may never have full motion again (though it is possible), but to optimize your motion and elbow function, early motion is critical.

Depends on frx: Generally we like to move elbows early they love to get stiff depends on frx stiff elbow is infinitely better than a loose elbow there are no solurtons for a unstable elbow.