I use 2,000 as a rule of thumb. The exact RPM isn't critical, just enough to get a good oil flow to the crank and rods, and enough centrifugal force to bathe the cam in a good volume. I usually adjust the fast-idle on the carb to about 2,000/2200, and just let it run at a steady speed. While it's breaking in, I check for steady oil pressure, final-adjust ignition timing, etc. This is also a good time to monitor the fill on the radiator, as it's not uncommon to have an air bubble trapped behind a closed thermostat. Once the stat opens and water starts to flow, top it off and install the cap. Nothing like an overheat in the middle of a run-in to ruin your day! It's important to bring the speed up as soon as you've confirmed oil pressure, to minimize the time the cam has to start scuffing the lobes. A good cam/lifter prelube, such as the blister pack Sealed Power includes with an engine assembly kit is a good idea, although any durable lube such as Lubriplate lithium grease will do. I used to use STP as an assembly lube, until I had several lube-related problems with new engines assembled with the stuff! Coat the cam lobes and lifter bottoms with the lube as you assemble. Motor oil doesn't seem to work well on a cam, as it doesn't cling long enough to protect the lobes during startup. Filling the oil filter with oil and/or running the oil pump with a drill motor and a priming shaft before installing the distributor also helps the oil pressure come up quickly.
BTW, I always pre-fill the filter on an oil change, also.
Jerry
