Ask HN: What's a fair share to give when adding a designer as a co-founder?

In the last 6 months I was working on stop motion app for Android. Now the app gets around 100 download per day in Google Play (first month) and around 50,000 picture was taken so far, and I'm planning to add paid version soon. But one thing I wanted is to add a designer as co-founder to take control of the user experience and the design but I wonder what's your thought about what could be a fair share for him/her. I know I'm going to spend a lot of time adding the new feature and maintain the app for at least 2-3 years and I wonder if a designer will spend around the same amount of time. (I meet the designer on the net and I only talked to him couple of times)