Like many of the lucky people above, I really like many aspects of my job, and technically I get three months off every summer already. So, it's what I'll do for fun when the day comes that I'm FI; it would take a LOT of money to get me to just leave and never be able to do this job again.
It seems like $4K/month (adjusted with inflation over time) would be plenty to live on the way I live now, including supporting my children and having some extra for traveling and the relatively expensive things I like to do, like downhill ski. I still use the same ski boots my Dad bought me when I was 12 years old; those are starting to not feel great and could use an upgrade :-)
What I would like, though, is freedom from the blah aspects of my job. The politics, the committees, the "extra" stuff I do to stay in the loop and be promoted. So, in a few more years when I hit FI, I'll be doing less and less of that stuff, and focusing on the fun parts of work. That's the goal, anyway.