On the embedded targets the "RT Get CPU Loads.vi" is the best way to get the cpu usage but it is instantaneous. Using it and calling it too often could change the system enough so that any data you were to get would not be accurate. Normally as long as the CPU load is not maxed out there is no reason to know the CPU usage over a given time. Could you please elaborate as to why you need the information?
It may be possible to use the Real Time Execution Trace Toolkit to profile your application and to see what is using the CPU and when. The maximum CPU load would not be directly available but the information from the toolkit might be more useful then just the maximum CPU.
Alex D
Systems Engineer
Academic Research
National Instruments