Even though you can go into the management in Exchange admin center and change the values to something higher or to use never it will actually not be more than 30 day retention time for the items in delete items box (or junk mail folder)
This is related to the Microsoft's global values. The management interface don't show any indications on the limitations so it's easy to be live that your settings are being active. You have to go to the Technet site to see the limitations values for different areas in Exchange online. You can find the page here
So as we can see there is currently a limit on 30 days, plus that items removed for deleted items will be available for 14 days as default, with a max of 30 days.
That's make it too a total of 60 days maximum before everything in your users deleted items are gone for ever.
The great news are that the Office 365 road map shows us that this limitation will be removed, starting from the january 15.
This doesn't mean that all customer will have the limitations removed on the 15., it will take some time before all customer have the limitation removed.
You can find the newest Office 365 road map here
Fellow MVP Jeremy Dahl have written a blog post on how to change the values, you can find the blog post here
I will still recommend too have a value on the retention time, but you can now (soon) decide yourself what it should be.