How long can I live in the United States with my German monthly income? I see different (more or less abstract resp. specific) approaches to compare wealth (of individuals, regions, and countries):


by living standards
by living expenses
by incomes/salaries
by purchasing powers
by price levels


One such approach – to talk at large – could be:


  How long can I – coming from country X – live in country Y by spending
  my last monthly salary converted by nominal exchange rate:
  
  
  as a backpacker or as a convenience tourist?
  adapting to the other country's standards which may be lower?
  adapting to higher standards than I am used to?
  
  
  How much does this period deviate from a month? 


My question is: Where do I find this approach taken?
This is tricky to answer precisely because governments vary a lot in the services they extent to visitors. There is also variation in the mix of consumption goods across countries as well as their relative prices. You might respond to local conditions by changing your consumption bundle more or less than the average person and that would change the economics for you relative to an average calculation. 

The simplest approximation is to take GDP (or the C component of consumption), measured using exchange rates rather than PPP, and divide it by the population. 

Think about it this way. Nominal GDP per capita in the US is about \$62,606 and in Germany is \$48,264. Assume it is safe to either ignore taxes or taxes are comparable in the two countries. If you have have saved the salary of the average German then you can afford the spending of the average American for about 9 months. 

Another way to do this is to look at destination country household final consumption expenditure per capita. As of 2016 this was about \$36,373 in 2010 dollars, probably about \$43k in 2019 dollars. According to Wikipedia, average German salary, net of taxes, is about \$32,000. So if you have a pension that pays net of taxes the salary of a typical German (also net of taxes), you can afford about 75 percent of an average American's spending. 

A third way is to use expatriate cost of living indices, although I've been told that these are more accurate for affluent urban households. For example, expatistan says that the cost of living is 53 percent higher in NYC than in Frankfurt. This suggests you can live for eight months in NYC for the same income that buys a year's worth of consumption in Germany.