Believe me, I would never dump the whole bottle in, but I am pretty sure I overdose, because I have a five gallon I do water changes on every week and it's really hard to get the correct amount for 1.25 gallons of water, if that's exactly what I'm changing, lol
Like I said, I small overdose has never been a problem for me. Using Prime, I dump 5 capfuls in when my tank calls for 4.5.....but when I used to use API Stress Coat(also dechlor), I'd squirt tons in, all the time...never had problem with that stuff either.
Depends how much you overdose I'm sure. I wouldn't go over about 3 times the dosage, but I have no facts to back that number up... just what I'd stay under.
I once dropped a 250 ml bottle of prime into my 29 gallon tank. I have absolutely no idea hoiw much spilled out, but im sure it was a lot. The tank and fish didnt react abnormally at all.