i used to keep one year, with auto-purge set. I had about 3,000 in PipesMagazine notifications, as I watch all posts.
Now, Outlook.com won't let me keep that amount, so there's 1,100 in Deleted folder.
Here's something frightening: At work, in my paid Outlook account, I do have it set to auto-purge at 12 months. I checked the other day and I have a tad over 168,000 emails in 12 months. When I complain about that, I checked my Sent file - it has 48,000 emails. No wonder I never get anything meaningful done.