[SpiralOcean]

My thought is... keep the archives forever. You'll probably have space on your computer so why not.

(I'm assuming you are speaking of the File->Archive function and not the backups that happen automatically)

To view at anytime, go to File->Open Archive.
To view items that you've completed, select the view filter and change the groupings to completed.

Or use the completed perspective that comes with OmniFocus. It may be in the customize toolbar section.

If you are speaking of the automatic backups, I would think a month is probably good enough.