If you keep up dosages like that for a few more consecutive days, then you could be presented with possible signs of liver damage. On two separate occasions, I was taking 7.5/750mg vicodin between 8-12x a day for spans of several days desperately seeking pain relief after exacerbations of my back injuries.
The first time, I felt "ok" until I suddenly felt the urge to chunder; and chunder I did. I spent about the next 8+ hours in the most miserable full-body heaving-to-vomit mode, violently throwing up every 20 minutes like clockwork. Needless to say, after the first two or so goes, I didn't have much left in me. Incredibly painful and largely unproductive bouts of dry-heaving eventually gave way to vomiting a somewhat thick dark brown and supremely bitter liquid with a seemingly fatty consistency. The first time that came up, I thought to myself, welp, now that I'm spewing raw bile, I'm one step away from sh*tting out of my mouth!
I told myself if I was ever in such pain again that I'd just go to the ER instead of taking so much vicodin. Well about a month later the pain was back big time so I went to the ER, and they gave me more vicodin, so what's a lad to do? Again, I spent two or three days (fewer than the last time) taking 8-12 of these 7.5/750mg pills per day. Then, my body had just had enough and it was the same story as the last vomit-fest, except this one lasted 12+ hours.
For about two weeks afterward, I took absolutely as little pain medication as possible. My liver was constantly aching and at times it was pretty severe. It felt as if I could delineate the boundaries of this organ in my body and so could tell that my liver was swollen and spilling over into places it shouldn't be. I wanted nothing more than to pull up the lower right side of my rib cage and remove this oversized blob. Once the pain went away, I can't say that I've experienced any noticeable lasting effects that might be considered signs of liver damage. But I guess I'd need a liver enzyme test or maybe a biopsy to figure that one out for sure.
I don't take my vicodin very often for a number of reasons, including these first experiences with APAP toxicity. I must admit that each time there were moments I thought that I had just about done myself in. If drug abuse overdose is going to do me in, the gruesome and protracted agony of APAP overdose is not
my preferred way to go about a premature personal demise that I'd prefer not to face in the first place.
When I do take vicodin, even just a pill or two, I usually do a cold water extraction - that's great advice from sekio. The N-acetylcysteine recommendation is also a good idea if you are concerned that you are undergoing an APAP overdose right now or expect that you will continue taking substantial amounts of APAP. I wish I'd had some.
Speaking generally, if you do experience APAP toxicity leading to liver damage, first you might experience the severe nausea and vomiting as I did or you might have no real outward symptoms at first. Then that liver/upper right quadrant pain sets in. I'm not sure how much damage the mere presence of this pain suggests, but typically a severe case of toxicity will follow a pretty standard course of liver failure over a few days and all that comes with that, e.g. death.
best, and mate, lose the APAP

x
edit: I should also like to point out that I was doing roughly your two-day APAP total every day for several days before I ended up with problems, and those problems didn't kill me, yet. So I wouldn't necessarily be alarmed.