In my experience, It refreshes almost right away....
I saw multiple posts regarding the refresh issue... But I can says that I don't suffer from that
Heres my setup
2x Seagate ST31000340AS as 2 volumes (NO RAID or JBOD)
On volume_1 I have a folder named Movies that everyone has "Read Only" access to
UPNP Media Server is activated and the folder selected is the same Movies folder has above
The Movies folder contains multiple Folders, one for each movie or TV Show
and each one of those folders contains 1 or multiple video files (mostly .AVI in DivX or Xvid)
If I had a new folder inside the Movies folder, and do a manual refresh, has soon has its done,
I can find it on my Xbox360 and play the movie... (the manual update can take a couple of minutes to complete, I have over 200 Folders and 1000 files in the Movies folder)
The only thng That I noticed is that the Xbox360 and a problem playing video that has a PATH and FILE name over 70 chars total....
I can see the Folder fine... (the name is truncated but its there) and inside the folder I see the movie file(again the name is truncated) and if I try to play it I get an error code...(don't remember it right now.. but its in one of my other posts here)
If I rename the Folder and File name to something shorter (that fits the 70 chars limit),
refresh the File List,
The new name is there and I can start playing the movie right away.