I do not believe that ancient bows were any less durable. For example http://www.grozerarchery.com/index_m.htm is a modern-day Hun who makes bows, both with modern materials, and in the old-fashioned way from thousands of years ago. While explaining the traditional method he offhandedly mentions that due to the quality of the glue, the bows will not become brittle or lose elasticity even after decades. That implies that they last that long, if not longer. There is also the fact that it takes a year to make a bow in the traditional way. If they disintegrated after only a few hundred shots, no one would have wasted their time. That would just be just a day or two of practice, and steppe nomads had to practice with them on a regular basis to become the astounding archers that they were.
Extra III. (composite or horn bow) finish: This is the kind that is the exact copy of the original, made with the same method as a thousand years ago, and consists of only natural components. The core of the bow is joined together of assorted kinds of wood. The inside (that is on the archer's side) is made of horn plating. The outside is made of tendon strings and fishbladder-glue, layered and shrunk onto the bow. These layers are sensitive to moisture, so it is covered with birch-bark or thin leather, which we decorate with original painted motives, on demand. The handle and tips are stiffened with horn or bone, and strengthened at the joinings with tying. The horn plating is made of the horn of Hungarian grey steer, water buffalo, ibex, wild sheep horn, and it's an expensive material, because there's not too much of it left. The fishbladder-glue can only be cooked from the airbladder of the sturgeon, a living fossil, that is also scarce these days. Actually, it isn't cooked, but we extract the glue from it with a "cold" process, soaking it in different kinds of materials. This way the quantity of the glue is small, but for that the quality is excellent. There is several reasons for using sturgeonbladder-glue for bowmaking for example: 1/ very strong gluing capability 2/ it won't get brittle or lose it's elasticity even after decades 3/ it is easy to work with. Obviously applying the tendon layer has got its own tricks. Before sinew must be soaked in various materials, impregnated, then only after this apply to the bow. This is why it is so unbreakable and resistant on the outer side of the bow. Besides, wood and horn must be treated, otherwise it would break easily in the bow. The horn-bow, the great invention of the Scythians, as well as the saddle, and later the Avar's stirrups. The rest of the eastern cultures took the knowledge of bowmaking from the nomads.
The making of a bow this way can take up to 10-12 months, drying of the wood not included.
» Thu May 03, 2012 5:34 pm 
I'm sorry I just had to click that banana) 
