The one I can think of off the top of my head would be to look at that agent install log on the client. in the TEMP directory of the user that the install was ran under you should find a WSCFG32.xlg file this should have a start and end time stamp in it. If the agent had been installed multiple times it might have multiple dates so find the date range you are interested in.
Other than that the reg key only contain over all agent install info not start and finish. Often the event viewer will show a finish time assuming it has not wrapped but usually does not have a start.
Jason