You'll have to build your trigger using LabVIEW since the DAQ card does not support it. The base package will be able to do this.
Basically, you'll need to do a continuous analog input to monitor the siganl, when it goes below your threshold value, reset your timer.
Have a look through the Examples that come with LabVIEW. (Help>Find Examples) Try one that does a continuous acquisition and add your timer and triggering to that.
Ed

Ed Dickens - Certified LabVIEW Architect
Lockheed Martin Space
Using the Abort button to stop your VI is like using a tree to stop your car. It works, but there may be consequences.