Bernd,
Not long at all, purely because when I was doing it, I prepared the bar to do another couple just in case something went wrong.
Because there was double angle being used on the outside, and it was a very complicated shape to measure up on the inside without a comparator microscope (which I don't possess), I actually cut slightly too deep on the inside for the first couple of attempts, but the third was spot on, with plenty of meat left on the inside.
The pic shows what happened with the first two.
BTW, between each one was only about 20 mins.
John
Added.
In fact the stainless bar (I have forgotten the material number, that was supplied by Bry) is still under my lathe, and still mounted in the chuck. Now that I know everything was successful, I can now take it out of the chuck and put it away somewhere safe just in case another one like this comes up.