I believe that it is due by March 31. It's either the 1st or the 31st, but I don't have the letter in front of me to check.
Here is the link to the form
http://www.vba.va.gov/pubs/forms/VBA-21-0517-1-ARE.pdfThe VA will suspend the benefit if they do not receive the form back by the deadline. And they don't care whether they neglected to send it to you or not. It's still your responsibility to send it back, so I would get it done as soon as you can. Getting a benefit unsuspended can be a nightmare.