Let me preface this with the following: I absolutely can not draw even if my life would depend on it, and before blender had zero experience with 3d software (I do however am quite comfortable with writing code).
For about the past one and a half years I've been messing about with and learning most of the stuff blender has to offer, ranging from just a couple of hours a week, to nothing at all, to every evening. The last 6 months or so of that has been spend creating an OC character completely from scratch (and the finish line is in sight jeej), of which some sort of diary is available
here. I estimate that I can probably create new female characters up to this level somewhere within a couple weeks to 1~2 months depending on the amount of asset reuse.
Can all of this be learned faster than that? Yeah probably, I've bumbled about quite a bit and had plenty of dead ends and failed experiments. But I still think about a year/half a year would probably be a safe bet (also highly dependent on the amount of hours you can/are willing to make).
To make things from scratch you have to become familiar with all the technical know-how involved (topology, shading, UVs, physics?, etc), how all of that applies to blender, and the artistic stuff. The first and second are just lots of things to learn, remember and a bit of experience in how/when to apply. The artistic stuff, i found, is mostly about making hours. Really, you don't truly know what makes a face until you've made loads that look just a tad bit weird because the proportions aren't quite right, or the shapes aren't the way you meant them to be. Luckily for us stick-figure connoisseurs you can keep sculpting until you get it right (or horribly wrong) and looking back can usually reflect on what exactly was off.
I certainly wouldn't say it was easy for me, and i wouldn't recommend it if your only goal is to make some smut. But if you want to learn 3d modeling in general/have some fun/just pick up a new skill set? by all means, have a go at it.