Recent Tasks Lifetime in vCenter Server

Once a task is complete in vCenter Server, the task shows up in the recent tasks pane for a default lifetime of 10 minutes after which tasks can be found in the Tasks & Events tab. The task list is capped at 200 tasks in order to allow for better scalability. These default values work well. However,
you can reduce the task lifetime by adding the following tag to the vpxd.cfg file.
<task>
<completedLifetime>600<completedLifetime>
</task>

Indicate the value of completedLifetime in seconds.
You can also change the length of the task history list in the vpxd.cfg file:
<task>
<completedMaxEntries>200</completedMaxEntries>
</task>

The vpxd.cfg file is located at:
<Program Data Folder>\VMware\VMware VirtualCenter

For More Info Read this http://www.vmware.com/files/pdf/techpaper/vsp_41_perf_VC_Best_Practices.pdf  (Page 20)