Comment 7 for bug 1321354

Microsoft servers drop idle connections after 15 minutes. This behavior can be disabled by this command as administrator (details: http://support.microsoft.com/kb/297684).

net config server /autodisconnect:-1

Alternatively, adding the following script to the gnome startup applications keeps the connections alive.

#!/bin/bash
while true
do
    ls /run/user/`id -u`/gvfs/smb-share:* &> /dev/null
    sleep 600
done