There's been lots of discussion about this for ABC engines in various places but the very high quality manufacturers (I'm talking things like Jett and Rossi) say to give at least 30 to 40 minutes on the bench with shortish runs followed by a cool down period and running rich. Of course what's rich to one guy is lean to another so the concensus was that if it was a little leaner than where it breaks in and out of a 4 stroke (by only a click or so on the needle) it was just about right. The fuel should have plenty of oil (20% or even more) and definitely some castor in it. The castor protects the rod bearings as they run in.
Use the smallest recommended prop to keep the revs up close to what you'll eventually fly at.
Personally I'd remove the backplate before you run it and flush everything out with fuel in case there's any particles left over from machining. And use an old plug for the running in...if any aluminium gets on the coil it'll ruin the plug.