Question: Completed Tasks

I have tasks which have comments inserted into them. When I click on complete, the task drops to the bottom of the list. That is fine. However, I don’t want to lose the comments and I want to keep track of the completed tasks. If I log in the next day the completed tasks are gone as well as my comments which I need to retain. Is there a way to preserve completed tasks for a certain length of time or have a setting where you can indicate how long completed tasks should be preserved on the system? Thanks in advance.