Rounding of the knives equates to narrowing of the individual cutting swath, so when to replace is dependent upon how finished you want the completed cut to look. If you're satisfied with less than a golf-course cut, a little rounding on corners of the cutting edge is acceptable. By design, there is individual cutting swath overlap that compensates in part for the missing metal. But at some point, individual knife wear progresses to the point where this overlap is compromised. That's the point at which the knives should be replaced. All this depends upon the original width of the knife and the degree of knife overlap. On the 917H I'm pretty sure the OE knife width (cutting swath) is around 2-5/8". When corner wear takes the swath below 2", I'd say it's time for new ones.
And when it becomes time to change the knives on a 917H, it's likely also time to change the hangers. The swinging knives wear grooves in their brackets. The deeper the grooves, the closer the knives swing toward the inside housing. When I bought mine, I replaced knives, hangers, hanger brackets, and the nuts/bolts/washers that held them in place.
//greg//