It's a combination. The Exchange server notifies Outlook of a server-side
change when the change occurs. Outlook then waits 30 seconds before it
downloads these changes. All additional changes that took place in these 30
seconds are downloaded as well.

As for sending changes from Outlook to Exchange; After every client-side
change Outlook waits 15 seconds before uploading these changes to Exchange.
If another client-side change took place within these 15 seconds the counter
will be reset. Theoretically this could result in an endless reset so a
forced upload will take place 60 seconds after the first change.

You can modify this behavior via group policies if you have a specific need
for your environment. More info can be found in the Office Resource Kit.