I'm not a maker of highly decorated rifles, but I aspire to be, when I have more time to get a few more builds under my belt. I've seen great makers at work and they vary in their speed quite a bit. Those who do it for a living get pretty quick and efficient, but any new design takes time to develop and execute.
Some builds take longer than others just to finish the basic rifle; as you mentioned, starting from a blank, etc, and then there are parts you could make, whether you had the bbl channel inlet, RR groove done and hole drilled, etc. So building the basic rifle can be done quickly by an experienced hand, and once a guy is good at carving or engraving, some work can be done pretty quickly. Other carving that is more complex, has more layers, etc can be challenging, and if you don't carve or engrave often, it is laborious and kinda nerve-wracking.
If I was up to speed, having recently carved another rifle with a similar, relatively simple design, I could probably do the carving and moldings on an entire rifle in 12-18 hours, but it would also add some time to my finishing. That includes lock moldings, toe molding, forend molding, some tang carving, carving fore and aft of the cheekpiece, and some squiggles around the entry thimble. When all the tools are sharp and I feel I am practiced up, I can do the carving behind a cheekpiece in 2 hours for example.
If I had to develop a new design, draw it, mess with it on the stock, maybe do a practice block or two of some details, and it was complex, maybe it could take twice that long.
Engraving is super-challenging for me because I've not spent the time on it that is needed. So i could putz around for 20 hours and not get much done.
Plain guns are no treat to build as any small flaw or lack of flow, etc stands out like a sore thumb. So in ways they take a lot of time to shape and finish too.