That's the way it works. Here's a simple explanation. There are two services running for hardware inventory: WMI and the SMS HW Inv agent. WMI (Microsoft's implementation of WBEM) is configured by default to collect hardware inventory every 30 minutes and write it to a repository (CIM.rep). SMS hardware inventory is a scheduled agent to collect information from this repository, and any .mif files, and send the information off to the CAP. Since it is scheduled, you only see it run for a brief time. It's the SMS client service that should be running all the time.