To Durk: Actually I was using the search function. I went back manually to in the thread list for some 40 pages. And I had studied the mentioned guides before I asked this question. I didn't find - and still don't - any hint. My idea is, there is some formula like:
(100-BuildRate) x NM-modifier (maybe capped?) x model-modifier (or family-modifier) x random values x other factors = BuildTime in days
These other factors might be general WS-production, Money-production or said production in the city of appearance. I think, in some post that discussion has already been conducted, but I didn't find it anymore. One example about the difference between prediction and reality: one hospital unit was predicted to last 50 days until completion, in fact, it wasn't finished before 90 days passed by. Anyway, thx for the reply.