Everyone will have a different opinion on this one
I've used Millers running in oil....good stuff! Seems to be widely used in motorsport.
Around 500 miles minimum seems to be the general concensus.
Make sure you don't let the engine run at tick over, or stand idling, or you risk glazed bores.(I'm presuming it's a new engine or and old engine with new rings/bores?)
Best bet is, start her up and imediately start driving. Take it easy until water and oil temperatures are hot and then give it some beans. You want to be on and off the power frequently, not staying at a constant rpm. I took mine onto the motorway and put foot to floor until around 6000rpm then off power completely letting the revs decrease to around 3000rpm, then repeat and repeat.
Again, when you stop, don't let the car stand idling.
What you don't want to do is drive it gently (but don't thrash when cold). It's the opposite of what most people think when 'running in'