Check Cashed in Error

I recently sent in an application along with a check for several hundred dollars for annual dues to a local organization. I was later informed that I did not get into the organization. When asked about what would happen to the check, I was informed by the organization president that the check would be shredded. I trusted him as this was a reputable organization and never gave it a second thought. Several weeks went by and last Friday morning as I was checking my bank account balance I noticed that the check had been cashed. I quickly found the e-mail stating that the check would be shredded and replied that the check had apparently been cashed in error and that I would like to know how they planed to return the money to me by the end of the day (I didn't ask that the money be returned by the end of last Friday, just that they let me know how they planned to get it back to me in a timely manner). Anyway, here I am late Monday night and I have not heard anything from them.

What is a reasonable amount of time to allow them to get the money back to me? What legal recourse do I have to force them to return the money to me? Can I demand interest on this money until they return it to me? If so, how much and how do I notify them of it (e-mail?)? Would this be worth pursuing in court? Any other recommendations on how to get this money back? If this was $20 or even $50 I'd be more willing to sit back for a while and hope they make it right, but since this is several hundred dollars I would really like to get it back immediately.

Any advice would be greatly appreciated.