Systems Management

Computers stay in inventory until you delete them or you have set the MIA settings to delete after a certain amount of days. 

If you are using 5.5 the MIA settings are found under Inventory > Choose Action > Configure MIA settings. 

Data Retention keeps uptime data, not deletes or anything of that nature. If you manually delete out of inventory or use the MIA settings, a record will always remain under the Asset Module for long term records or historical tracking.

Hope that helps. 

look under control panel - settings - data retention