When can you return to work with a fractured 5th metacarpal?

Depends. The answer depends on what you do andhow the fracture is treated. It usually takes 4-8 weeks for a fracture to heal so if you use your hand at work especially to lift or other heavy use of your hand you may be out of work for a while.
Depends on the work. People with "sedentary" or "low demand" jobs can often return back to work following splint immobilization of the injury. If you perform desk-type duties, the splint can adequately protect the fracture from displacement. People who engage in heavy demand activities, such as construction work, may need to avoid full work duties until the fracture has healed completely.