which logs and how do you read them....

How much more is hard to say, but usually if you check the logs you will be able to check for given system how much has been backed up so far. Difference between that and total size is (around) value which still is missing backup.

daemon.log on server. Read them with your favorite editor.

I assume the saveset is not 5.6TB but that refers to the total data? If so you could monitor which groups have completed which will give you an idea of how things are going.

Also, if this is full backup and amount of data doesn't change too much you can check mminfo to see value that will get backed up (usually that value is less than one one showed by system tools).