Hi,
Pretty much the same as a flint cock. Measure from the center of the tumbler shaft hole to the center of the percussion cone (the cup at the end of the hammer).
It’s important to know if you are replacing a hammer. If the throw is different than the original, it may not hit the nipple correctly. Same with a flint. It needs to hit the frizzen face correctly to get good spark and not chew up flints.