More specifically, what is the single metric you would use to monitor personal wealth?
The number of days you could sustain your current lifestyle if you stopped working for money today
The metric I prefer is net worth, minus the value of your home, then divide by your annual expenses. The house is subtracted because you need to live somewhere, so its worth isn't part of retirement savings. I divide by expenses to create result that really answers how close one is to being able to retire. The target is to have 25X your required spending gap. Note, as you close in on retirement, and social security is still in place, you can use it in your planning. If I were in my 20s or 30s today, I wouldn't use it in my numbers.
Net worth in local currency.
The wealthiest man is not the one who has the most, but who needs the least.
Wealth is not what you earn, nor is it what you spend. It's the difference between those two.
StackExchange points, obviously. =)
I think it should be the amount which makes you feel satisfied, strong and brave for the future.