the number is different for everyone - but as the world bitcoinises, $40k/yr is not going to buy a good life anywhere (gov subsidised energy, healthcare, etc will all go away), nor will it take $1mm/yr to live well in the west
Discussion
I think $40K per year will still buy you a relatively decent life in most non-western countries, for at least the next 10 years. Of course, it depends on what kind of lifestyle you have, what you health looks like, if you have any dependants, etc.
I think the answer to this question depends a lot on one's age too. The older you are, the more you probably care about the monthly income than waiting for massive Bitcoin gains in 10-15 years.