Number of seconds before an unused index can be closed automatically.

Use AutoCloseTime to have the cache automatically shrink when idle. On each access to the cache, the last-used time for each index is compared to autoCloseTime and, if the index has not been used for a sufficient period of time, the index is closed and removed from the cache. 

The cache does not have its own thread to implement AutoCloseTime, so it only checks AutoCloseTime when the cache is accessed for any reason.