Being able to make Space Marines is a labor intensive and well guarded secret to those who possess it. However, given that the Chaos Legions don't get their reinforcements solely from renegade and fallen Astrates chapters means they have some understanding of how to make their own. At the very least we know Fabius Bile makes quite a name for himself selling not only Astrates but "enhancements" to the Chaos loyal Astrates.

The question then becomes, "How much time, material and resources would be required to create a Space Marine? Should it even be possible for a PC to obtain the ability to make his own Astrates?" And even deeper, "Is it possible to reverse engineer the process back to the point of creating Primarchs given enough time, materials and resources? If so, is such an endeavor within the scope of standard Black Crusade game?"

It should be possible to create new Space Marines but very, very difficult.

Reverse engineering the process to the point of creating Primarchs would be basically impossible, unless you could get your hands on the Emperor himself, or MAYBE one of the actual Primarchs. It'd be basically impossible without one of those options to base your project off of.

As for creating a normal Astartes? Well, naturally, you'd need gene seed. That's the big issue, as it's well protected by all who hold it (this shouldn't be an acquisition test, ever, it should involve a lot of roleplay to acquire). As for time? I imagine it'd be a lot of trial and error, so it'd be a number of months at least. And the process wouldn't exactly be foolproof; not even the apothecaries of the Iron Warriors have managed to make a perfect method, most of their subjects fail to accept the gene seed and end up horrible mutants.