As CEO of Getaway, I rarely get a phone call or a text after hours and seldom do folks expect a reply from me late at night, on weekends or on vacation. I think, if you asked around the company, you would find that this is true not just for me, but for most of us. You would find lots of exceptions, but you'd also find that off-time is generally respected. We have developed norms that are very different from many other "always-on" companies.
As you start a startup, a lot is rightly made of establishing your vision, mission, and core values early. I'd also urge you to actively think about the behavioral norms you'd like established in your company. As with those other foundational elements, it is exponentially easier to do this when you are at your smallest. It may seem silly to talk about how a few people should interact, but what you are actually doing is establishing the implicit contract that future co-workers opt into and planting seeds that produce powerful cultural crops year after year. It's like teaching young kids about responsibility before they have any real influence: Steering to the right path early avoids tough or impossible course corrections later.
Norms are by definition not rules. They are generally the way people inside your company should behave. "Don't spend company money on personal stuff" should be a rule. "Have a written agenda for every meeting" might be a norm. Breaking rules should lead to discipline or dismissal; breaking norms should be expected as long as the pattern remains intact. Like values, norms are powerful because they give folks a good sense of what to do without being explicitly told.
Amazon famously has a norm around writing six-page memos for every big decision, and silently reading them at the outset of a big meeting. In business school, I was taught that a difference in norms contributed to the "worst merger of all time," of Time Warner and AOL: The Time Warner folks all defaulted to phone calls, while AOL employees used email. Norms dictate whether the "big boss" in any given conference room mostly talks or mostly listens, who gets promoted, where work actually happens, how fast or slow things move, and thousands of other things big and small that we end up labeling company culture.
One reason to actively think about norms early is that they get established whether you think about them or not, and some norms can be destructive. What started as a few beers after work for the co-founders can turn into a party culture (see WeCrashed). Long, collaborative work sessions in the early days can turn into a decision-by-consensus culture that is tough for those who think best alone. Early "scrappiness" can set an example that corner-cutting is always good.
I'd urge you to write down your norms right after you establish your values. For each value, agree which concrete behaviors you believe will guide the team to living up to that value. For example, if, like Getaway, you have a value along the lines of One Team that implies transparency, you might have a norm of making time for unrestricted Q+A at most meetings. Norms should be measurable enough that at the end of any given time period, you can ask your colleagues, "Are we doing that consistently?" and they should be able to confidently answer yes or no.
One of the things I learned the hard way is that it is difficult to write norms in an environment in which there are many different types of roles: We have employees who are full-time, part-time, at HQ, front-lines, who work inside on laptops, and who work outside in nature. It is OK to have different norms that are relevant to different groups, as long as they aren't in conflict.
Remember that you won't make everyone happy, and in fact that is a big part of the point. By establishing strong norms, you will help your organization grow into what you want it to be by being clear about who you are and who you aren't. If you are clear about that, you will better attract people who will thrive in the environment you want to create and dissuade those who may struggle. If you have your norms clear from the jump and communicate about them honestly, you will save yourself the real pain of attracting people who prefer to work differently than your team. That clears the way for you all to focus on the important work.