I don't know the exact date also and I want to know it too but if you based and look to their road map ICO crowd sale will start on May 2019 so that before or on May 2019 private sale will sure come to end.
The exact date for the closure of private sale wasn't specified in the road map. In my opinion, I believe the private sale can run indefinitely until the target is reached. Its very likely that the private sale will run simultaneously with the public sale. That's better to give the private investors enough time to carry out their research before making their commitment.
Even as much as I don't fully agree with you concerning the private sale running indefinitely, I think it will be logical for the team to fulfil their goals in segment. It's obvious that getting $150m hardcap is almost impossible, but I believe if the team can fulfil some portion of the project and raise more funds subsequently to fulfil the others, then the private sale could be carried out in streams.
Am in total agreement with your view, this was actually what was meant by indefinite private sale. The project has different features, the cloud computing, mining and neuroneum card. It could be difficult to actualize the three different features simultaneously, that explains why I believe that each feature could be fulfilled independently with the available funds while sourcing for funds for others.