You can upgrade online at (http://www.ni.com/upgrade). 6.0 is the current version so there is not a currently available upgrade. As for the free upgrade timeframe, there isn't a timeframe. We have free patch upgrades and major upgrades that are not free (currently $395). Free upgrade version would be like 6.0.0 to 6.0.1 or 6.0.2. Major upgrades would be like from 6.0 to 6.1 or 6.0 to 7.0, etc.
Hope that answers your question,
Chris Matthews
National Instruments