Whilst there may be no overall limit, at the folder level there are
recommendations as in http://support.microsoft.com/?id=905803. Quoting:

"You can help avoid poor performance in Outlook by carefully managing
the number of items in folders, especially the Outlook folders that are
heavily used. These folders include the Inbox, Calendar, Tasks, and
Sent Items folders and any other heavily used folders.

The recommended number of items in a folder depends on several factors.
These factors include the client's proximity to the server, the storage
infrastructure, the load on the hard disks, the number of users, and
the number of restricted views. We recommend that you maintain a range
of 3,500 to 5,000 items in a folder depending on the capacity of the
Exchange Server environment.

The following list includes ways that you can help manage the number of
items in folders:· Use folder hierarchies to help keep the number of
items in a folder to the recommended values.
· Use mailbox manager policies.
· Use client-based archiving solutions.
· Use server-based archiving solutions.
· Use mailbox size limits. "

The one that sneaks up on our users is the Calendar.  Five meetings per
day at 240 days/year "only" allows two-three years worth of data.  If
your company has pack-rats they'll want to keep everything forever.
(Even worse if they keep attachments on their calendar - and even worse
for other users opening their calendar as a shared calendar!)

John Samuel