When first installing Archive Manager version 4.6 and only specifying 1 drive for the new Full Text Index the Rollover Policy will default to 180 days.
The rollover policy is tied to the number of drives. 1 drive = 180, 2 = 360, 3 = 540, 4 = 720 days.
It is forward-only and not retroactive
This can be changed in the WebUI under Administration > Index Rollover Policies by adding additional drives.