I worked these kind of hours:

a) when I developed my first software product and began selling it online ~20 years ago, and

b) as a consultant for a company experiencing a steady stream of fires

Each spanned about a year before settling into a more reasonable pace.

In the first case, I was finishing university and bootstrapping my company on a shoestring budget. I convinced a couple friends to help out here and there, eventually hiring one as my first part-time employee, but until then I wore all the hats. If I had to do it over I would probably drop out to focus solely on the startup, but I expect that just means I'd allocate my time differently not less. No regrets, and I look back on this as one of the most productive periods of my life. I wound up with rock-solid software (manageable enough to support on my own) and an autonomous sales pipeline that repaid my investment of effort with dividends - in time I could arbitrarily take days off for impromptu ski trips after huge snowfalls, and I recall a few where I got a notification part way through the day about a new top-tier sale from someone I'd never heard of (whoot! drinks are on me).

The second was at a company which had, before I got there, oversold its woefully incomplete and buggy enterprise platform. My role was a mix of product and (to some degree) project management, customer account therapy, and mentoring developers. This time I wasn't the only one working outrageous hours; the whole team really stepped up when I came on board. Their previous boss was a VP injected by the company that acquired theirs, a total jerk who gutted morale. The survivors were happy to have someone who actually stuck up for them, outlined a vision to sanity (even if it was imperfect) and was willing to occasionally jump into the trenches and code alongside 'em over weekends to meet a deadline. Part of that stint was at a fixed rate, although I smartened up quick and renegotiated hourly. The difference had no impact on my work ethic (the only change was at one point the CEO dragged me in to complain I was the highest paid person at the company including him - hah!)

Both ordeals definitely entailed sacrifices. Work-life balance wasn't a thing. I developed acute RSI during the first stint, before I learned about ergonomics. Though interestingly, I was physically more fit during these insanely busy periods of my life, as the high level of stress prompted me to run ~3 miles each morning as an effective coping mechanism to stay sane. My brain and ego thrived off the pressure and chronic sense of urgency, although at times it felt wearing and I knew I'd burn out if I tried to maintain it indefinitely. I couldn't imagine attempting this with a family, and the lifestyle certainly delayed any plans there. But one of the devs I worked closest with put in similar hours and he's a father of 5 boys (no idea how he managed).

It allowed me to "not work" for extended periods in between - i.e. take some long vacations, spend time exploring experimental new tech at my own pace, loads of pet projects, even pick up new skills totally outside my wheelhouse. Maybe that fabled work-life balance just played out over a longer timescale...

Now, instead of fighting it, I lean into my natural ebbs and flows of productivity. I confess I wear both experiences like a badge of pride, and my inside-voice silently mocks all you lazy 35-40 hourers ;-)