Some kits mention in the online ad, that their experience with customers is that a kit takes about 100 hours to complete.
My first kit, a Kentucky .45 pistol in percussion, took me about three weeks of off-duty work...
Then, the Kentucky Rifle, in .45 percussion, took me about 7 weeks of off duty time to complete. Both shot perfectly!
I Browned both. Did NOT sand them down until they were very skinny in their wood profiles.
Then about 10 years, later, refinished both, taking down the wood a teensy bit more... They were shooters, still looked great,
but, I wanted a darker finish on the wood.
Now, I will be doing a .54 caliber Perscussion set. Style might be in a "Plains Rifle" style with a matching pistol...
I intend to shoot deer and hog in primitive season with them.
There is a whole group, or sub-culture if that's the proper term, of builders who start from scratch building muzzleloaders. I suggest that you pull up the Contemporary Longrifle Association website and "blow your mind"! The sky is the limit on what you can get in the way of components, but be prepared to spend a little money IF you get top-of-the-line components. You might consider starting with one of Jim Chamber's kit guns....rifles or smoothbores. There is not a kit made and sold that is up to Jim's quality, and the training/experience that you would gain going this way would be excellent training for total building with individually purchased components. Pull up the CLA website. You'll like what you see, and while you're at it pull up the CLA's official pub, American Tradition magazine summer 2013 edition, and look on page 46. That's my .32cal squirrel rifle at the very top of the page, and it was made from scratch like you are wondering about.