What is the grace period when paying a developer on spec?

You would have to have a very convincing argument why your idea is so good that the developer should take the risks that

1. your idea will not succeed in the marketplace (most don't), and that
2. you will actually pay up,

given that

1. you aren't prepared to put money into the idea up front, and
2. the developer doesn't know you.

You might be able to mitigate this by giving the developer a significant share of the equity but, unless the developer knows you (directly or indirectly) the inability of the developer to judge whether you will be able to deliver is likely to prevent an agreement.

Any decent developer will laugh for days on that proposal. Why should he do that, instaed of writing his own software in his time and sell it, or earn some good money? Note that your idea of good money VERY likely is his idea of really bad pay.

Your idea, your marketing, your risk.

It is highly unusual. Actually I can't think of a single time where I have seen this done for software development.

Just to illustrate the culture: There are some lines of business where unpaid summer internships are common. In programming internships are common enough, but they're almost always paid internships.

I doubt you'll find any programmers who will accept your "on spec" deal; and I'm certain you won't find any good programmers who will take those terms.

So you want to sell all the rights to the app code and not just the app on the Apple store? Unless you can start getting potential buyers lined up with some type of documented commitment from them, I doubt any developer will go for this.

I don't see how you are convince them of that unless they are a partner with partial ownership of your company.