So I just got my engine running, it has new flat tappet cam and I did the initial break in. I was told I need to wait a couple hundred miles before full throttle and any high rpms. How long does it need to properly break in?

 

Technically, change the oil and filter after the break in and drive it however you want.

If you want to get specific, you break in the cam for about 20 minutes. Change the oil and filter. Drive like an old man for about 500 miles. Change the oil and filter again then drive however you want. That process is generally the break in for a complete rebuild including rings.

For just the cam, the 20 minute break in is enough. Just change that oil and filter again to remove any traces of break in metal that may contaminate the bearings.

 

Extended gentleness is extremely helpful to making new valvesprings last. Other than that, I'm all in favor of driving it "like-you-stole-it" immediately after the 20 minute oil / filter change. This "harshness" is actually better for engines that will always be run hard. But if it's a stock rebuild for your grandmother, then do the 500 miles gently.