There's a lot of "it depends" here. As both a former founder and early startup employee these are the things that come to mind—
How far out did you discuss this with your employer? Were they aware of it when you joined? There's a pretty big difference if this is 6 months or two weeks away. Setting expectations go a long way here on both sides.
What is the size of the business? People are more critical the smaller the business is. The stakes are very different for a business with 10 vs 100 vs 1,000 employees.
Is the business cashflow positive—or near it? You said startup, so that makes me think they aren't cash flow positive. Three consecutive weeks is a lot to ask for a small business that isn't making money.
Essentially, the more stable the business is the less of an issue I'd expect it to be—given expectations were set in advance.
The point is that some arbitrary number of weeks of vacation isn't the business-destroying choice this founder makes it out to be. Use your PTO. You earned it.
It depends on how important you are. If you are the director of sales, software architect, or something like that, it can be business destroying; particularly if some unforeseen emergency happens. An example is a new big client is suddenly requiring something happen before signing a contract and everyone needs to be on deck for that to happen.
If you are one of 100 CSRs, then it wouldn't be business-destroying obviously.
Any serious business needs to have a continuity plan and can't depend on availability of a single person. What if the person you depend on will have a medical emergency or resign?
Business owners can take this risk but using "don't take more than 2 weeks of PTO" as a risk mitigation strategy is disingenuous and disrespectful to their employees.
Startups running lean, which is what you need to do in order to maximize runway, don't have the money or time for "continuity plans".
Haven't you ever joked about the "bus factor"? All the startups I've worked at would have likely failed if any one of the core people had to depart, for any reason.
I've yet to see a job where hiring a new employee is more efficient than letting one have off 3 weeks instead of 2. There is a really good chance that a new employee wouldn't be working yet before they've returned.
I gave a measured reply given what the poster provided.
"Unbelievable nonsense" is a pretty harsh response. If they are working for a 10-person business burning cash, that is very different than a profitable 1,000-person one.
The poster asked about etiquette. Personally, I'd be offended as a founder or employee at a 10-person company if someone asked for 3 weeks off at the last minute.
Not to mention, just about every position in an early startup is mission critical. I'd say 3 weeks off would be fine in the company I work at, but I'd be expected to be contactable and available if an emergency happened. We're about 5 years in though. The first few years, I really couldn't take vacation other than a 3-4 day weekend here and there.
How far out did you discuss this with your employer? Were they aware of it when you joined? There's a pretty big difference if this is 6 months or two weeks away. Setting expectations go a long way here on both sides.
What is the size of the business? People are more critical the smaller the business is. The stakes are very different for a business with 10 vs 100 vs 1,000 employees.
Is the business cashflow positive—or near it? You said startup, so that makes me think they aren't cash flow positive. Three consecutive weeks is a lot to ask for a small business that isn't making money.
Essentially, the more stable the business is the less of an issue I'd expect it to be—given expectations were set in advance.