It all depends on the scale length of the guitar and the length of your head stock. I imagine the average is about 20 inches or so.
I start with a 36" neck for most of my 3 strings. I do a 25 1/2" scale length for which I use a 19" fret board.
For a first build or "early" build it's easier to simplify.
You're probably using a standard 6ft. long 1"x2", so cut it in half 36" which either gives you two potential necks, or one neck plus extra wood for box reinforcement.
Either way just about any design or plans you might be following will work out fine if you start with a 36" length.
You need to pick a scale length, most common are 24", 24.5", or 25" (Gitty sells templates or you can copy one from an existing guitar, or if your feeling really crazy you can try to measure them yourself from a spacing guide.)
But basically allowing 4 to 6 inches for a headstock and depending if your design goes through the opposite end of the box by an inch or two, and of course depending on the length of the box. You'll get somewhere between a 19 to 24 fret neck (or the equivalent length on a fretless neck) with the 36" starting piece.
Of course you can cut a specific neck length based on scale and the size of the box in order to achieve a specific number of frets, but I wouldn't worry about that until you have a few builds under your belt.