1/8". Have a look in the toolboxes, there's often a piece of sheetmetal with a bend in it to use for setting the gap. If there isn't, just pickup a small piece of 1/8" from any metal shop about 1" wife and 8" long, put a bend in the end so you can get it in there.
Of more importance than gap is having a crisp square edge on the anvil. You can flip it if it's rounded, and they can be repaired by building up with metal and brought back to square with a flap disk on an angle grinder and a steady hand. A crisp square anvil will put out great chip, keep your blades sharp longer, throw the chip further and make the last little bit of logs self feed. A rounded anvil will do much the opposite.
