All I can say is that it's de facto possible to create a GmbH after you already have started building something that will be owned by the GmbH without requiring a formal transfer of the assets. To do so in a way that is legally safe and sound you should speak to a legal expert.

As I understand it, it's less that there's a legally defined period, it's more about the limits of what you can rationalize or to what degree history can be redefined. If all the code you've written is sitting on machines under your control, who's to say that the code you wrote before the company was founded is the same as the code the company ends up owning? Especially when it was just a draft prototype you definitely threw away anyway. If sale of the company or investment ever comes up, they'll do the due diligence to make sure the documented history is up to par anyway.

If any of this was too subtle: legally, history consists of what everyone agrees history is. Founders can be expected to bring in their prior knowledge when founding a company, so any software produced by them for the company while employed by that company is owned by that company, even if it happens to be indistinguishable from software they wrote prior to founding the company, as long as there are no third-party claims to that previous software (which would create liability for the founder anyway). If you're the only person who knows what color your bytes are, well... .