According to Eric's formula, 6 meters, but I think that the formula fails when there is little or no "free-fall" in the dive.
Assuming that your DNF is about 30m, then your CBNF limit should be no greater than 15m.
So somewhere between 6 and 15m

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I am going to guess that you should be able to do at least 10m and might be able to do as much as 15m -- beyond that, you start to get a little heavy at depth and the amount of effort required to get back to the surface goes up pretty dramatically.
It would be interesting to compile a list of stats from people and then use a curve-fitting program to get these kinds of estimates. My guess is that a 2nd order curve would give pretty good results. The CAFA site, FreedivingSpots.Net and most meet results have data that could be used for data, and Excel has curve fitting add-ins.