The orcs in my world are humans/dwarves that have been exposed to an airborne mutagen that permanently denatures the myostatin receptors in their skeletal muscles. This stops their muscles from responding to myostatin, which eventually makes them far more muscular than a regular human or dwarf. I have a character who has been brought up to think of them as lumbering, rabid demons, and has (forcibly) been exposed to this; once he discovers this, how long does he have before he's no longer "human" (or considers himself as such)?
As a rough ballpark figure, I have estimated two weeks, based on a brief look at this paper. However, that was for mice and required two weeks of inhibitor injections. Is this time frame plausible for a human, after exposure to an airborne inhibitor?
The only real criteria I have is that it has to be in reasonably real time; not overnight or even over a day or two, but quickly enough to be obvious that it's happened.