The short answer
As early as you can — the sooner, the better. Start with a full-time product design ‘all-arounder’ with strong visual design skills. Someone strong in interaction design, visual design (typography in particular), information design, and prototyping. Then, they can pinch-hit or advise an agency on a brand design until it requires a full-time hire.
The less short answer
It is hard to imagine a software business that wouldn’t massively and fundamentally benefit from having a great designer on the early team. Each likely has several layers that must be understood, desired, purchased, and ultimately used by people.
The rationale for hiring early is to accelerate the growth of a product development organization that sees itself as a three-way partnership between Product, Design, and Engineering. Design should be a stakeholder and a contributor to the roadmap, and helping to deepen our understanding of customers. Designers should be prototyping and iterating closely with engineers. And critically, Design should drive the union of the product and brand experiences as into coherent whole.
The earlier you do this, the more the culture is oriented around partnership and collaboration. This is a compounding investment that takes time to tune and mature. It delivers value immediately, but the massive long-term value of lower design/product debt, design-oriented growth and sales drivers, product usability, customer love and trust, higher retention and engagement…That’s what we’re aiming for.
The longer you wait, the greater the Design debt will be. Paying this down later is increasingly expensive, as the product grows in complexity. Design debt comes in many forms: a product that is poorly understood by customers and buyers, valuable features that are underutilized, or aren’t surfaced in the right places at the right times. An information architecture that is difficult to follow, or hard to expand, or unintentionally prioritizes the wrong things. Eventually the lack of a design system causes the front-end engineering process to be slower, less performant, and more bug-prone. The cost of grafting a design organization (and a design culture) onto your team at later stages is far higher and more difficult. You’ll likely wish you had done it sooner, or from the beginning.
There are internal benefits, too. When the product and brand are well-designed, it is a source of inspiration and pride — a rallying point for everyone at our company. It increases a broad sense of ownership. It inspires the team to continually raise the bar because it is a tactile example of quality. That sense of quality should bleed into everything across the business.
The design team should become one of the company’s best ‘sense organs’. Because design sits at the intersection of all of the business units and functional organizations, we have a unique viewpoint that helps detect opportunities (and disconnects) across the entire landscape. Designers are usually the first ones to spot duplicative work, or product and brand concepts that are in opposition to each other.