I've only built a dozen or so wheels but I probably would've stopped at about 120Kgf. My 32h Open Pro's are tensioned to about 115Kgf drive side rear and about 105Kgf on the front. I only weigh 150 lbs. but I've beat the living tar out of them and they're still true.
Anyhow, on an perfectly evenly tensioned wheel it's usually the spoke bed that gives it up and not a spoke. Gerd Schraner states somewhere in his book that good spokes can typically take up to 200Kgf. For that reason I usually look for the minimum spoke count the rim is commercially available for. If it's an old 36 spoke single wall rim, I stop at something less than 100Kgf. 32 hole double wall at 36/32*100 or 113Kgf, 28 hole double wall at 36/28*129Kgf, etc.
I think 129Kgf would be the start of the danger zone with Open Pros but I'd like to hear other people's opinions myself.