The first time I breeched a barrel that took me about 20 hours - I was so worried about doing it wrong that I think I miked the plug after every light stroke of the file.
Now it takes an hour if I stop for a coffee break a couple time during the process.
Likewise, the first couple of butt plates I installed took "many" 5 or 6 hour evenings - then I found how to "custom fit" that last little gap with a "hammer"
The time will also depend on what you have available for tools. A precarved kit does have much of the work done for you, but you could still piddle around for quite a few hours with an exacto knife when a dremel would make it a 5 minute job (but could turn into a many hour job if there is an "ooops").
As most have suggested, if you are "confident" and possess at least some skill (with wood/metal), 100 to 150 hours is probably realistic.
Unfortunately, sometimes life gets in the way which is why some/many kits go "unfinished" for years.
Be realistic as to how much time you can spend on a daily/weekly/monthly basis. If it will sit for weeks until you can get at it again it will take "more hours" than if you can spend a couple hours "regularly" - because if it's hit/miss you will spend time reorienting yourself with "where your at", you might forget that you needed to do "this next" and end up back-tracking etc.
So if you are looking to have your rifle by say 1 September, can you spend 10 "uninterrupted" hours a week on your build between now and then?