I fall somewhere between the CTO and CEO camps, wearing both hats but definitely preferring the technical one when a more competent CEO already exists - so my schedule is going to be more of a "maker's schedule" than a "manager's schedule". That means long blocks of focused work time.
Typically, things are fairly calm up until the actual launch (but I'm of the belief that you're cutting things too close if launching a week later changes the fate of your company). Once the launch happens and you start getting traction, you will forever have a feedback pipeline to address from your users, and will have to balance this with other responsibilities that also increase with traction, such as bizdev. You do live and breathe the company for a while, but I think the people putting in 100+h each week, every week are destined for burnout before they see anything come of their efforts. It's a marathon, not a sprint, after all.
Instead, I like to optimize for efficiency and spaced repetition. Back when I was working on an app (also when I was writing my thesis), my day used to look something like this:
~10 AM: Wake up, shower, dress, feed my cats, etc. Record any ideas I had overnight.
~10:30 AM: Check emails, reviews, web alerts, and analytics dashboards.
~11:00 AM: Check to-do board, get to work on the highest priorities. Stare intently at screen. Write some code. Compile, pet cats. Repeat.
~2:00 PM: Lowest energy time of the day for me - so it's lunchtime / time for lunch meetings. I try to make as many meetings as possible lunch meetings, because they put both people in a good mood, require fewer context switches, and because I am an unabashed foodie.
~3-4 PM: Still in lunch / meeting mode. Any phone calls get booked during this time. This is also time for people who want a "quick coffee". Check email, etc. again.
~4 PM: By now, I'm getting back into the thick of work. Headphones on, music up, IDE open.
One hour before sunset: go for a walk alone or with someone I care about, followed by dinner, sometimes shopping or a movie.
~9 PM: Update todo board, then back to work until the wee hours. Such a good time to focus.
~2 AM: Update todo board for tomorrow, get at least 7h of sleep.
Overall, I probably worked about 9 or 10 hours a day. But importantly, I varied the work up and used the natural high and low points in my circadian cycle to my advantage. I seldom felt tired or bored, the defining characteristics of my day when I used to work for other people. Occasionally I'd be glued to the screen or the phone before big launches or partnership/acquisition discussions, but I made sure these were the exception, not the rule.
I sold the company off for a decent sum before it had the chance to grow into a larger business that required an office, fundraising, coordinating lots of employees, and a bunch of other things that seemed like they'd destroy the lifestyle for a questionable amount of incremental value. When there were tasks I was weak at or didn't have the time to do effectively, I hired contractors or a firm, but only through personal recommendations from people in my network.
These days I'm involved in an education nonprofit, and things are a lot lower intensity - so I spend most of my day tinkering on side projects (particularly IoT stuff) as the mood strikes, with the occasional paid engagement from a friend who needs something built. I do sometimes worry I'm losing the discipline I previously had to a regimen that's too unstructured.
So that's during and after a modest success. Rocket ships, VC-backed startups, or companies with really frenetic founders would have different characteristics, naturally.