So, assuming a human in plate armor got caught up in a fine mist of this acid (roughly 65% of the body is covered, head included), what injuries would they suffer and how fast would their incapacitation be?
I'm going to go for "instantaneous incapacitation, death within two minutes at the most".
The fine mist would inevitably be inhaled, leading to edema of the airways within seconds. An immediate tracheotomy is the only way I can think of that has any hope of salvation for the unlucky knight. Suffocation would probably do him in before shock has any real chance.
You can (reasonably) easily find images on the Internet of people who had about 10-20 cc of commercial sulfuric acid thrown at their faces, what is known as vitriolage. This quantity is often (80-90% of the cases) survivable, but even with immediate medical care brings loss of function and significant disfiguration. And that, to repeat, is the effect of a quantity two orders of magnitude less than your dragon can spew, usually thrown with much less proficiency and delivered in a significantly more inefficient form (inhalation of sulfuric acid vapours is almost invariably fatal; even the accidental inhalation of what vapours may effuse from a vitriolage attack can be fatal, and even when it is not, it leads to potentially life-threatening respiratory damage).
Defense against such an attack would require a way of sealing the armor so that, for a couple of minutes, the knight can breathe the air trapped inside. Not something to look forward to, given the average knight's reported level of personal hygiene, but survivable.
Then, something that can easily be sloughed off - maybe a mantle, or a large composite hoodie. Impregnated with water and alkali to neutralize the acid as much as possible, with a quick-release brooch. Maybe more than one mantle, in layers.
The best strategy if the dragon's attack cannot be thwarted or avoided entirely would be to draw the attack in such a way that it can be more easily defeated.
The knight would for example goad the dragon, assuming (as @JohnDvorak correctly points out) the dragon is not clever enough to see through the ruse, then quickly pivot and fall on his knees closing the visor of his sallet, leaving exposed the back and the (jettisonable) pauldrons. The back would be defended with a thick mantle of leather and maybe gold foil. Once the acid jet has been exhausted with certainty, the dragon can be battled with much better chances.
Dragon-killing knights would also very quickly employ a different kind of armor: some kind of transparent protection (quartz?) for the eyes. Probably a sealed or sealable helmet. Possibly a thick but still light leather suit instead of steel armor, to increase speed. This again assumes that the dragons don't also have, say, strong claws, in which case abandoning armor would not be advisable.