4 answers
Salesforce archives activities using the following criteria:
Activity archive criteria:
Events with an ActivityDateTime or ActivityDate value greater than or equal to 365 days old
Tasks with an IsClosed value of true and an ActivityDate value greater than or equal to 365 days old
Tasks with an IsClosed value of true, a blank ActivityDate field, and a create date greater than or equal to 365 days ago
I believe, however, that they still count towards your data usage. What I do is a Task cleanup every 6 months. That is the only way I am aware of cleaning them up. I would suggest you build a report to see what types of tasks are the biggest culprit and figure out a way to reduce the amount of tasks that are generated.
Here is a link to a thread on the same topic - https://success.salesforce.com/answers?id=90630000000gt1RAAQ
