Apologies for the double-post, but I just did some durability testing via crafting.
First, I loaded myself on stone chunks. Then a chisel. Then I started with the stone hammer and made some of the small stone blocks; the one's that make 20 blocks per craft/chunk.Made all I could, and the stone hammer broke at 20 chunks used. Then I repeated this with the iron hammer and war hammer (separately, of course) and each broke after 60 chunks used. So, now we're at 140 chunks used. Chisel's still going strong. Repaired (with basic repair kits) all three hammers: The stone took 1; the other two took 3 each. I then junked the iron and war hammers. Repeated the crafting, then repairing process until the chisel finally broke. 60 more chunks used. That's a total of 200 chunks/crafts. Repaired it, and it took 10 repair kits.
So, from this data we can assume that the durability of stone hammer is 20x, whereas x is the amount of durability used during crafting. (Frankly, I don't see the durability being 20, since I've swung the iron hammer at stone blocks for more than 60 swings before it broke.) And the iron and war hammers are 60x. As for the chisel, well, it's 200x. And as for the basic repair kits, we can safely assume that they repair 20x durability each.
TL;DR The chisel will last 10 times longer than stone tools, and ~3.3 times longer than iron tools in crafting.