To me, First Fire doesn't really count as part of the break-in, it's merely the final step in the assembly. I get it started, pick it up to about a thousand rpm, let it run until it has a certain 'hot' smell, never letting the speed settle, up and down a little, senses very busy. Mebbe three minutes, max. Oil is 10W-Something; thin. Take lots of time to correct any problems, let the thing cool between runs (and I mean at least an hour), change the oil at least twice in the first six hours, and doing anything else you can think of to ease the birthing pains.
Then, I re-check the valves and timing; anything that might have kicked loose; anything I might have overlooked. Then I refill it, fire it up, warm it up . . . never takes long with a new engine . . . bring it up to about 2,500, hunt around that speed range while I'm crawling all over the thing. It's on the test stand, I'm not wearing ear muffs. I've got all the sensors cranked up to nine; smell, sight, hearing, hearing, hearing, touch. I overload in about fifteen minutes, have to shut it down and think about it. I might run it a little more to get more data on a particular thing but most often drain it, let it cool down. You always throw away this oil; it's your first break-in cycle. Check it for chunkies of course. You're real interested in any drips at this stage. There shouldn't be a single one so if you see one it usually means the thing goes back into the shop, gets torn down. This is about the worst thing that can happen. And it does, but no oftener than once in every hundred engines or so. And yes, it's usually my fault. Just plain damn stupidity or carelessness, like letting my cuff drag across the parting line just as I'm closing the case; dumb stuff. All that work ruined. But I'm getting better at it :-)
I usually keep an engine on the test stand until the rings are well seated and the temperature comes down; about six hours, on average. Then too, I tend to build 'tight' engines; you have to be careful breaking them in, giving them plenty of time to cool down between runs, keeping the oil changed even with the filter installed. After a couple of hours you and the engine are pretty well acquainted; they're all a little different. You've got the carb(s) tweaked down to the fine numbers, the thing will start on the first revolution of the crank, the case is dry, the blow-back is next to nothing and it's starting to develop its own unique sound that will allow me to identify it years from now, assuming the muffler and intake are the same.
An apparently mindless part of breaking in an engine is seating the rings. We call it Bumps & Grinds. You slowly open the throttle to about 2500 then drop it, just let it shut. Anyone hearing you would think you're just playing with the poor thing, up and down, loud then quiet. You'll be wearing your ear muffs by then; you're breaking it in, not checking it out. You never do any of these things for very long; mebbe 20 minutes at the outside, more like fifteen, then shut it down and go do something else; let it cool off. If you're doing more than one engine you'll have to dismount it hot, put it back on its scooter, put the next customer in the chair. (I've done as many as five engines for one customer, all as a series. Five is mebbe one too many for one man at a time.)
You keep records. If you're using a test stand you'll usually have a Hobbs meter; a kind of clock. If not, just your watch and and a note book. Doing one engine, you can't get too confused. The engine will start flattening out its curves after five or six hours of running. Fuel consumption will have dropped down to some steady figure, as will the temperatures. Oil pressure will have come up. You'll want to verify that with a wet & dry compression check and a leak-down test, if you've got the harness. You will have to take a last look at the valves . . . unless you're running hydraulics.
The complete wear-in cycle continues for about a hundred hours, after which the engine's curves will stay substantially the same for the next thousand hours of engine operation. Understand, the engine is still wearing, still experiencing friction. But now it is wearing-out, not in. The wear during its service life will be very uniform and consistently small, but after a thousand hours or so you will see the first signs of terminal wear from the valves, the weak links in the VW system. The lower end should be good for at least 1,500 hours and will probably run 2,000 without a whit of trouble assuming you're running a full-flow filter. Beyond that, it will depend on if you've got a shaft seal, how well you've done your maintenance, your particular flying habits.
Breaking in an engine with hydraulic lifters is a little different than with solids, first because the valves probably won't tell you anything at all. They'll always be running a perfect zero-lash. But hydraulics in a freshly machined magnesium crankcase can generate a bit more 'mud' than with solids, or even hydraulics in an aluminium crankcase. So you take it easier at first, accumulating about 45 minutes of run-time before you start seating in the rings. Understand, the engine has never been allowed to sit about at an idle. Breaking in an engine means wearing-in an engine; it is a kind of controlled friction. But the mud does bad things to the minute clearances of hydraulic lifters, which are having an especially hard time because the engine is running hotter than it ever will again. So you do more short runs and longer cooling-off periods when you break-in a Type I fitted with hydraulics. That is, during the first two to three hours. After that, there doesn't seem to be any difference, except the hydraulic engine will run quieter. And a little more efficiently, but you won't see that until you've got mebbe six hours on it, by which time it probably will have been installed in a vehicle and roared off down the road.