Brief Answer:
Depends on how large it is.

Detailed Answer:
Thanks for using the Ask a Doctor service.

I will have to ask you to provide the details of the situation. A magnet is a solid insoluble object and even if it is ingested, it will not be digested (although the acid in the stomach may corrode it to some extent). Needless to say, it will not be taken up by the system. It will pass through the gastrointestinal tract unaltered (provided that the size is not large enough to get it struck mid way) and will pass out with stool. Therefore no symptoms are expected. However, in the event that the size in big enough to get struck in any part of the gastrointestinal tract, symptoms of intestinal obstruction will occur. Abdominal pain and constipation will be the fist symptoms in that case. Management in that case would be surgical.

Regards

---