Literally never.
And if you are talking about more of a "stache" calculation--invested or other assets that will be used for living expenses in retirement, I suppose the true answer there is also never, but about 95% of that is in the portfolio trackign spreadsheet I use, which does have a formula and a tiny box way at the bottom that adds everything up, along with a few other calculations (like the amount it will be in x years at y% growth and w years at z% growth). I added those just because when I was building the spreadsheet I was having fun including every bell and whistle. But I don't check those every time I update the sheet, and a few of our stashe assets aren't included (like our two rental properties, for example).
I just don't care, especially about net worth. If I sold my car and my tv and my sofa and my computer, I'd have $X. But I don't plan on selling any of those things (and in fact will replace all of them when they die) so what good does knowing the value of X do me? And as for my stache amount? It's nice to have a rough idea of what things look like, but retirement for DH and I is based on some career goals for him, not on getting to a specific number. By the time he reaches those goals, there's almost no way (outside of a catastrophic market collapse) we won't be at well more than the stashe amount we need. And fluctuations in the value of that stache don't trigger any actions according to my personal investing plan, so what's the point?