As soon as you can reasonably afford it ( if you can't pay, go get another cofounder ). The web has evolved from simply working to working seamlessly. Consumers expect a certain level of aesthetic and usability.

Design is one more element of your product, just like solving a real problem and doing what it's supposed to do.

I would prioritize a designer as much as an engineer. I have seen a huge difference I product adoption when the product is complemented with visually pleasing aesthetic and good usability principles.