In general the LMS 4.2 runs VRFcollection jobs and Summarizer jobs periodically. When you hit a high number of jobs (15K if I'm not wrong), you start to get an error message on Job browser page. Then the purging may come in handy.
Go to [Admin] > [Network] > [Purge Settings].
To purge the jobs related to VRF, see the topic "Purging VRF Management Reports Jobs and Archived Reports" on the following document.
I have programmed a purge for jobs related to VRF, but I can't do the same with summarizer jobs; the page doesn't load.
I have been deleting some jobs by cli. I have seen that I have many periodic jobs that Ciscowork do every hour (CPU utilization, availability, etc). I have much more than 15k records. I don't know if I can't delete these jobs or at least the older records, becuase I think taht this is an automatic LMS action.
Sometimes a java process that purges the summarizer jobs gots stuck and an error message appears. Try to restart the application (net stop crmdmgtd && net start crmdmgtd - for windows - or application stop/start LMS - for linux-based appliances) and then run the summarizer purge again.
Maybe you will have to runs a few more purges to get an acceptable number of jobs.