I can't give you an exact number, but I can say less than an hour. This is one of my remote employees, so I haven't seen the problems with my own eyes. The reported problems were failure of attachments to add to the email, then I was told that emails were not going out and were just sitting in the outbox. I then told her to stop using ZD for now and switch back to webmail.
I was really hoping this would work because some of my employees are off network a lot, and it would help to be able to work on email while offline.
It will be really hard for me to get you more information because the employee is remote and not super technical for fetching logs and what not. Unfortunately I don't have any Vista 64 machines available for my use (they would be running linux anyway)
Thanks!