If you're a solo founder and decide to add a co-founder at a later stage as you build the business, what should the equity be at these different stages:
What would you say is fair equity % to give to a co-founder at each stage?
There are many factors at each stage that would affect the way equity is structured. There is no hard and fast rule by which you can decide until you reach that stage.
If you're just starting out, I would recommend reading Joel Spolsky's answer here: Forming a new software startup, how do I allocate ownership fairly?