Office 365 will delete your subscription data after 90 days and no later than 180 days after cancellation. However, what happens if you have a requirement to delete that data sooner?
Microsoft offer something called 'expedited deprovisioning', you can read about it and the normal cancellation and deletion processes here, Expedited deprovisioning should probably be exposed more in the article - but at least the article now include information on the matter.
So if you have a requirement for a more resolute data deletion process, contact Microsoft and ask for expedited deprovisioning. They will delete your data within 3 days this way.
However make sure you have backed up the data you need or migrated it elsewhere as this is a permanent process and you won't be able to get the data back afterwards!