My total income (full-time work plus nonwage income) first hit the $75k mark back in 1997. That same year my nonwage income exceeded $10k as the stock market was booming following my increasing purchases into stock mutual funds.
I used a lot of my gains in those years to pay down 2/3 of the rest of my mortgage, with the remaining 1/3 of paid off in early 1998. This big reduction in my monthly expenses was a big step towards being able to RE.
However, it was in 1997 when I started to get burnt out at work. I actually worked PT for a few months in 1998 before resuming FT work. I would actually max out at just under $100k in 2000 counting nonwage income, as my level of happiness dropped from being burnt out.
In 2001, I would cut my pay by about 35% to switch to PT and boost my happiness. However, as my income kept rising again in the next 6 years, but only to about $68k (nearly half was nonwage), my level of happiness was dropping again due to further burnout even from PT work.
In 2008, I had enough of the burnout and PT work, so I took a $300k payout from company stock and retired. My income for 2009, all nonwage in my first happy year of being ER, is under $40k, about the same as my total income in 1988, coincidentally another very happy year for me.
So perhaps $75k is a big turning point for me?