The scan rate for the SCXI modules is set by your data acquisition card. Once you determine which DAQ board you own or will buy, you can view the User Manual at http://www.ni.com/manuals. Look in the Appendix for Specifications and you wil find the scan rate. You board will be able to scan at any rate up to the maximum as defined in the Manual/Datasheet.
The scan rate is set in software. If you are using LabVIEW you will find many shipping examples which demonstrate how to perform data acquisitions. The scan rate is a parameter to the AI Start VI.