I think it's reasonable to want a full backup every so often: most of my machines are configured to do one every few months. There's nothing magic about that number: the right value is going to depend on how much data you have, how fast it changes, how likely you are to want to restore from anything other than the most recent snapshot, how much traffic and storage costs you, and how paranoid you are. Other people might want a full backup every week.
Unless you do a full backup from time to time the archive size and recovery time will continue to grow.
I don't think duplicity specifically has a "check" command http://pad.lv/660895, but it would be nice if it did. It is very prudent to do a test restore every so often.
A related question is whether you should keep more than one backup chain. Again, it depends on the cost. One reason to keep one is that you could restore from it if the current chain is corrupt, either because of hardware failure, OS failure, or a duplicity bug. Of course if the old chain is very old, restoring from it may be of limited value.
Making a full backup always uploads a full copy of the data.
If the client concern is the fraction of bandwidth used, rather than traffic charges, you might want to run it under eg trickle.
Best Answer
The following command is probably what you're looking for:
example:
Found primary backup chain with matching signature chain: ------------------------- Chain start time: Mon Mar 4 10:37:27 2013 Chain end time: Fri Mar 15 15:42:22 2013 Number of contained backup sets: 3 Total number of contained volumes: 10 Type of backup set: Time: Num volumes: Full Mon Mar 4 10:37:27 2013 8 Incremental Fri Mar 8 15:53:07 2013 1 Incremental Fri Mar 15 15:42:22 2013 1 ------------------------- No orphaned or incomplete backup sets found.