There is a vi called "Measure Pulse Width or Period.vi" under " data acquistion----->counter"
Should be able to use it to measure a pulse width. (at least at low frequenecy (just refer to th help..)
Nidaq 6.9X is required.
If you for some reason installed Tradtional Nidaq (Nidaq 7.x) it will probably not work since traditional Nidaq does not support the PCI-1200 series