Alright, now that I have a little more time, here's a longer summary.
The answer to your question is simple. There is no answer.
On any sword, you want an edge hardness of over 50 RC, 55 RC is better, IF the sword can handle the stress. The spine can be anywhere from the same hardness (a through-hardened blade), to an extremely low hardness. In a massive generalization, the harder the edge, the softer the spine will need to be to prevent shattering.
However, the greater to the extremes you go, the more fragile a sword is. A 65 RC edge with a 30 RC spine will be prone to chiping at the edge and bending, a terrible sword.
As with everything, balance is the key. What do you want? If you don't mind sharpening as much, a lower edge hardness (53-55) will be more resilient. A harder spine will be springy and flex instead of bending (flex = return to shape, bend = stick). However, too hard anywhere and you run the risk of breakage.
As I said, all about balance.
However, more important than any of these numbers is the quality of the heat treatment. If the blade has an even hardness along it's length, then it will hold very well. However, if it varies, you create the weakest link effect. If the rest of the blade bends, the stress can accumulate at the hardest spot, causing critical failure. This is the number one problem with cheap production blades.
Fencing Club at UH