I think I did 6 rustings on this, but it may have only been 5. At about 4, I could have stopped, but went one or two more 'for good measure'.
I did a CZ82 several months earlier and only did 4 on that one. I've had it to the range several times, so there are a couple hundred rounds through it since the refinish and, so far, there is no (zero) wear marks from the recoil spring on the barrel (it's a blowback operated semi - the spring surrounds the entire barrel). No wear anywhere else, either. This gives me a great deal of confidence in the toughness of a rust blue finish.
Of course, the durability and number of rustings necessary may vary greatly depending on the composition of the steel. I think I've been pretty lucky so far. I do try to be as careful as possible (for me). I always use distilled water, and I de-grease after every carding with acetone. One other thing I've taken to doing is heating up the metal (with an old blow dryer) before applying the rusting juice.