Hi There,
That's an excellent question that primarily depends on what type of activity you're referring to.
Marketo will automatically clean-up data for inactive anonymous leads and some activity log entries after 90 days. It does this to ensure the performance of your Marketo instance stays speedy as you gain more and more data. Anonymous leads with no web activity after 90 days after automatically removed - these leads generally are unlikely to return to your site with the same cookie. Marketo also automatically archives activity that isn't as useful following the 90 day period that would show up in the activity log, including change score, change data value, visited webpage, clicked link on webpage, and synced lead to SFDC.
I would take a look at this article (Understand How Marketo Archives Inactive Anonymous Leads) for a more detailed explanation.
If you're worried about losing this type of behavioral data I would recommend exploring interesting moments to highlight key activities in your CRM/Marketo or sending over certain types of activities to your CRM for historical recording.
Does that help?
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