In the beginning, there was an e-book...

This feels like one of those 'how long is a piece of string' questions to be honest. Depends on the book, the method of distribution, etc, etc. You should definitely have a figure in the contract if you ask me.

I know of one publisher that used to offer print pub after 250 rather overpriced downloads. They've since dropped all print publication, leading me to suspect they found less risk/more money in going all-e.

A lot of epubs that offer print do so as essentially another format. Trying to get books into b&m stores is massively expensive and risky for a publisher with an epub business model based (it nearly bankrupted Ellora's Cave, from what I've read). A lot of stores requiring offset printing, deep discounts and returnability, all of which are costly and require considerably more capital than ebooks.

The usually requirements that you'll see involve the length of the book and a certain number of copies sold (or money made, which is essentially the same thing). Some epublishers do it automatically after a certain length of time. If the publisher is looking to produce print editions, the requirements should appear in the contract and they should be specific, not open to interpretation.

Even the publisher may not have a specific figure in mind, which kind of makes it impossible to tell.