Qualified written request information needed

In order to send a qualified written request I've been told that it must be sent out within thirty days of receiving notice of default. I'm now behind about eight payments on my mortgage, and in the judicial process of being sued by my servicer. I have been served by a complaint by a sheriff, is it to late to send a qualified written request disputing the debt? I'd like to send a debt validation letter disputing the entire debt upon proof of claim... I'm just not sure if there is a statute of limitations on sending a QWR to dispute the debt? Can anyone provide me with a specific answer to this?