is there a limit for job suspension?

What is the name of your state? Ohio

Two weeks ago I was placed on unpaid suspension for what the company called "suspicious behavior." I was not doing anything illegal or that should warrant getting fired, but the company is allowed to fire employees at will. I was told I would be called regarding the status of my job, but to date I have received no call and no letter. My benefits are still in tact, however. I am in a union. Is there a limit to how long a company can keep you in limbo? What should I do from here?

Is there a limit to how long a company can keep you in limbo?

No, there is not. They can leave you on suspension indefinitely.

What should I do from here?

Cooperate fully with any investigation; answer truthfully and politely any questions put to you; if you have heard nothing in another week or so, call them up and POLITELY ask about your current status.