How long do you have to wait to request a book after the one you requested went missing.  Or do you automatically get your request put back on your wish list?  Thanks.

If it was a wishlist book, you automatically go back to your previous spot on the list.

HOWEVER, you should not immediately request another copy (PBS suggests you wait at least 2 weeks in case the book has just been delayed by USPS) because you might end up with 2 copies and minus 2 credits (if you receive the "lost" book you HAVE to mark it received from your transaction archive unless the person mailed it in the week before it was declared lost or later).

I always check where the book is coming from.  Books that travel over water are often declared lost before they arrive.  So HI, AK, GU, PR, and all the types of APOs will just take that long if they were sent by media mail.

IIRC PBS guidelines suggest waiting 2 weeks before reordering, which means you might need to go to your wishlist and put it on hold after the system moves it to 'lost' so it doesn't automatically re-order.

From Help documents:

What you should do if a book you requested is declared Lost in the Mail:

• Contact the sender, to find out if mailing was delayed for some reason.
  • you can do this from your Transaction Archive, using the PM button on this Lost transaction in that list.

• If the mailing was not delayed, the sender should call 800-ASK-USPS or go to http://postalinspectors.uspis.gov/forms/MLNtRcvd.aspx (the Postal Inspection Service's website) and input the information online there . This will help USPS try to recover the book if it truly has been lost in the mail.
• Don't re-request the book for at least 2 weeks, unless you are willing to run the risk of getting two copies (and paying a credit for both).

If you do end up with 2 copies of the same book, you can always post one of them (assuming both arrive in postable condition).  You may get lucky and get that book quickly requested from you.