How often are my IMAP e-mail folders synchronized?

How often are my IMAP e-mail folders synchronized?

For POP3 accounts, you can set the time interval for automatic mail checking in the Settings dialog. IMAP account folders are synchronized every time you view their content.

If you don’t want the folders to be automatically synchronized (for example if you use GPRS connection), then click File in the main menu and select Work Off-line.

I disagree… IMAP is not fully synchronising and certain messages (even when marked for download) are not being downloaded evern when online!