You can hook it up right away....as long as the rings are seated which happens quick your fine. I prefer to put 500-600miles on any new motor before you balls to walls put it on kill though
Start off with some short pulls on the N2O shutting the car off and pulling a plug to read it. That is the only true way to read the plug....
...and unless you have an awful lot of experience reading plugs, I'd suggest you use some careful dyno pulls with a wide band to be sure your mixture with nitrous is ok.