I don't really have a number, per se; I'm playing it by ear. My quest for FIRE was inspired mostly by professional burnout, which has (oddly) faded away since I no longer feel trapped and have FI in reach. With the desperation gone, I think I'll have more than I need to retire by the time I feel like quitting. I've been thinking I may direct a bit more disposable income to charities and other discretionary non-consumption spending, still saving around 50% but slow-rolling the final transition.
The "magic" number is definitely under $1M... I haven't figured out how low, partly because even after 18+ months here, I'm still learning (and implementing) new fixes to lifestyle creep. On top of that, I've become pretty comfortable with leveraged rental investments (higher SWR%) and will probably stay in real estate for fun (adding an easy $5-15K with minimal labor).
I could almost FIRE now on my part-time income, or on a $300K stash and just one of my two PT jobs. But I love my safety margins, and I like the idea of leaving a huge legacy to a good cause, so the number itself... may not matter in the end. Then again, if my job does a 180 and becomes an existential nightmare once again, I'll get way more serious about the magic #, which will hopefully turn out to be less than what I have by that point.