Infinity. If bitcoin is mass adopted then the value of the current currencies will fall to zero. If the value of 1 dollar is zero, then it would take an infinite amount of dollars to equal one bitcoin. it will be measured as an infinite amount of any fiat currency to equal 1 bitcoin. This is the end result of hyperinflation.
In 1913 you could buy ten loaves of bread for $1. Today you can barely buy one loaf of bread for $1. Probably you can't even buy the whole loaf for $1. If the us dollar inflates further, it might cost you $10 to by 1 loaf of bread, later it might cost you $20 for 1 loaf of bread, it wouldn't take much longer after that for it to cost you $100 for a loaf of bread, and if the ball really gets rolling, you might need a wheel barrel of dollars to pay for just one loaf of bread. At that point how much do you think 1 bitcoin would be going for? A billion dollars? A trillion dollars???
Eventually it would go to infinity as the value of a dollar drops to zero and at that point it no longer would make sense to measure the value of 1 bitcoin against dollars. It would make more sense to measure the value of bitcoin by how many loaves of bread it would buy you. Or perhaps how many Lamborghini's it would buy you, etc.
Soon it would not take any thought about the value of a loaf of bread, everyone would know that X amount of bitcoin is how much a loaf of bread is worth, and at that point bitcoin has become the new unit of account, replacing the dollar or any other currency in that role.
So in conclusion, at mass adoption the value of 1 bitcoin is infinity.
