Business Development, like Marketing, should be brought on when the product is ready. That usually means that there's a great understanding of your core customer, the use case, and a business model that scales.
I would suggest doing atleast 3 business development deals first to test the model, perfect the process (i.e. comp structure, requirements, marketing assets, agreements, etc) - then if they work, you can bring someone on to go after other identified (similar) opportunities.
Just because you have a lot of inbound opportunities to partner doesn't mean you should - be sure to focus on the product, API's (they are the future of BD) and business model to ensure they can support a successful partnership.
No sense in doing a campaign where you loose 30% of your margins if you only make 15% = You're loosing money on those deals.
It happens way more often then you think.