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.