The online Accuracy Calculator was setup to
address this type of question: http://www.ni.com/measurements/accuracy/
The accuracy you can expect will be improved by averaging your readings, using a range that most tightly fits your signal, and calibrating your board often.
You should have an external calibration performed on your device at least once a year. http://www.ni.com/calibration
You can also
perform an internal self-calibration as often as you like. Here is a KnowledgeBase entry about self-calibration in LabVIEW.
The link to the Application Note talks about
self-calibration using NI-DAQ functions.