If your upload queue is empty and SpiderOak reports "backup complete", then the sync will occur very quickly. SpiderOak backs up before it syncs, so if there are items still to be backed up, then SpiderOak will finish encrypting and backing up those items before it performs any sync. This is by design to ensure that syncs always preserve all versions of your files, and to protect you from data loss even in the case of failure. How long this will take depends upon the amount of data to be uploaded.
For this reason, syncing is not always instantaneous. Speed is one point on which our less secure competitors will beat us, because they don't take the time to encrypt and back up your files before syncing them. They are faster; we are secure and private.
The SpiderOak icon in your computer's system tray or notification area will indicate by changing color when it is working on but has not yet finished an action such as uploading a file to be synchronized. The Dashboard tab of the application will give further detail, particularly whether or not uploading and synchronizing has completed.