What's the default time ? 20 min, 1 hour....once a day?

There really isn't a default time. BES syncs on demand, when there is a need to sync.
Changes are reconciled almost immediately.

What platform and BES version are you running?

The BES server polls the mails db every 20 seconds (default) and scan for new documents (mail and calendar) But there's some low priority docs that takes more time to sync (address book and deletions if i remember) can take up to 20 mins

... on Domino, yes this is accurate. Not for Exchange.
In an Exchange environment, BES does not poll the Exchange server.

Oh didn't know that
Thanks for the information!

4.1 Exchange on Windows 2003 box..