Well, there's patient and there's patient
On my connection, I'd expect 500MB to download in less than an hour (often a lot less, depending on the server).
Zimbra was downloading for 7 hours, then it seemed (from the log file) to stop downloading anything very much, but carried on trying.
It would be useful to know the experiences of others with similar volumes of mail: if it's a general issue, I'll raise a bug.
It would also be useful, for the long downloads, to have some sort of progress bar or, even better, to allow you to use the Zimbra desktop and sync. in the background,