If you’re talking about last time the table was updated in terms of its structured has changed (new column added, column changed etc.) – use this query:
SELECT name, [modify_date] FROM sys.tables
If you’re talking about DML operations (insert, update, delete), then you either need to persist what that DMV gives you on a regular basis, or you need to create triggers on all tables to record that “last modified” date – or check out features like Change Data Capture in SQL Server 2008 and newer.