Hopefully someone with Hasselblad experience/expertise can answer your question regarding the H3DII-39 and it's limits. However, it is quite safe to say that 15 minutes is very far past it's capabilities.
The only digital backs spec's to exceed 4 minutes are:
Phase One P45+
Phase One P30+
Phase One P25+
Phase One P21+
Phase One P20+
Of these the P45+ is generally accepted as the king of long exposures and the P30+ a close second. All five backs are rated to go to 60 minutes of exposure at base ISO in average temperatures (less when it's hot out, more when it's cold out). I've not personally done any long exposures with the P20+ or P21+ but I can say from personal experience the quality of exposure of a P45+ or P30+ at 15 minutes is still very very good. All five take automatic black frame subtractions and apply those black frames automatically in Capture One.
With digital backs, exceeding the rated maximum long exposure of any given back (any brand) drops quality very quickly. For instance a P65+ is rated at 60 seconds and is quite good at 45 seconds, usable at 60 seconds, poor at 75 seconds and very bad at 90 seconds.