I've been FIREd for going on 4 years now and I rarely look at my portfolio. But, I keep my finger on the general pulse of things and there is some question in my mind about when I might need to take some action.
As an example, when I REd, I think the cFIREsim or whatever I ran had me at....was it 98% success rate? So there are some scenarios I could fail and some scenarios that counted as success that would not be so great to actually live through. But I always knew that those scenarios had me being totally passive. And if the market tanked IRL, I wouldn't be passive. I'd cut some expenses, like my extravagant budgeting for travel (well, I ended up cutting that without wanting to the past couple of years), the significant portion of my budget that I give to charity, etc. And of course, the more quickly you can tighten that belt if those bleak markets come to pass, the better your ultimate outcome.
So I do feel an obligation to be responsive if things are getting quite bad, but also not to overreact to normal volatility. The kind where cFIREsim would still have me dying with $18MM in the end, which is a bad outcome in the opposite direction (for me). None of us are clairvoyant, though, so it's hard to know which scenario this is. Right now it doesn't feel like something I need to take action on. But I honestly don't have a specific number or formula for when it would get to that point.