From about a month's worth of forced study in December. But Rommel's knowledge and approval of the plot was perfectly well known and has been for years. I don't know where you get your information from either, but one note in the Wiki article on Ludwig Beck reads:
In May 1944, a memorandum by Field Marshal Erwin Rommel made it clear that his participation in the proposed putsch was based on the precondition that Beck serve as the head of state in the new government.
Further, Rommel wasn't even particularly coy about it to his staff in the days prior to the July 20 plot. I'll happily provide other sources that verify this if you like - they're about 100 miles from where I'm sitting at the moment but won't be difficult to dig up since they're sitting on my coffee table at the moment.
He never really spent much time in Berlin once the war began. Either being in Africa, Paris, near the fortifications on the English Channel. Since he was hardly in or around the presence of Hitler most of the time during the War, there would be very little to gain by having him involved or being aware of the conspiracy.