As you are aware, you have years of build up of the ferrous particles.
The B or second stage is going to have to be repeated a few times. no need to go to the C or third and final step until you have the ferrous build up removed. Just allow B to dwell/set for the alotted time,(see more info at the end of this post) agitating with the VG-IS pad.(you may wear it out on this job), when the B has been on for 10 minutes or so, rinse off with plain water, wipe the excess rinse water off and re-apply the B, working it with the pad.
This is going to be your longest and most labor intentsive part of the job.
I am sure that once you have removed the ferrous, due to the amount and size of the rust blooms, there will be marring and a good polish job will be needed.
So, follow the main instructions, keeping in mind that you will need to "modify" the dwell and number of applications of the B part, repeating it a few times to desolve and lift the ferrous out of the paint.
Do not fail to do a final wash with the C-Detailing Wash, as it is necessary to neutralize the B that is down in the paint pores.
Can't give you an exact time for this. I did a car for GM down in Mexico a few years back that had set outside, no washing, nothing, for three years, next to a steel recycling plant south of Monterey. It was like yours, white but looked like a tan/brown.
It took me a bit over two hours and was a real toughy, as this is not a normal condition for vehicles.
Just don't rush, and since you will be allowing more product on and in the paint, do not do on a hot surface or in direct sun as it will heat up the paint/metal and may create too much reactivity of the B.
Also, don't be afraid to "up the dwell time" of the B, just keep it wet, not allowing to dry on the surface. You should be able to go up to 12 to 13 minutes of dwell time per application of the B.