Landscape timbers are made form a wide variety of woods - longevity will depend on the specific wood used and if they have been treated (many have been chemically treated for durability). For untreated timbers or softwoods, rather than roofing tar use a regular wood preservative, like is used for decks and wood fences, and coat the entire timber. Also sinking the timbers into concrete rather than just digging them into the soil will give you a much sturdier fence structure and will extend the life of the wood.

I needed a very cheap fence once and sunk landscape timbers in concrete. One year later, one could easily break them off at the ground. So they were somewhat more temporary than I had hoped for. I did not treat them in any other way, but they came "treated" ... theoretically.

gardengal and yardvaark are totally right, landscaping timbers are no good if they are not treated for weather erosion. Your best bet is just go to a reclamation yard, purchase some cheap used wood panels and create your own fence.