It also depends on which parts you are talking about. The cabin cover, wheel and leg fairings, wingtip fairings, etc for my 2020 vintage kits are excellent. There is some cutting of pieces to fit (doorways, for example, are made with more material and you have to cut everything down to a final fit.
Fitting the doors into the doorways takes a fair amount of work - trimming to get them exactly how you want them. And the curve of the doors didn't exactly match the curve of the cabin cover for me, so I have been gently building up the cabin cover with epoxy and cloth in little bits so that they are aerodynamically in line with each other.
Those are the "finished" parts - some cutting to fit each other, and some filling (like the backside of stabilizer fairings).
The unfinished parts require more work. There are some curved fairings where the landing gear meets the body and wheels where you have to add a few layers of fiberglass cloth to make the final shape of the unfinished pieces match the shape of your parts. These take some time. I've spent maybe 30-40 hours on these four pieces recently. Added small squares of soaked cloth while cleco'd in place, then I removed the fairings and added bigger pieces of cloth on a workbench, then I added three layers of epoxy mixed with micro and cab, which I massaged in place with gloved hands. Then hours and hours sanding the rough shapes smooth, filling in the small bubbles and low points with more epoxy, sand, fill, sand, fill, sand, fill. Almost done now - I'm finishing with (so far) 4 layers of a sandable filler primer to really smooth them out. Then will sand again.
Similar story to how you have to build your own fairing from scratch where the lower edge of the windshield meets the top of the front of the plane. 10 layers of fiberglass cloth in various widths, then several layers of epoxy "mud", with lots and lots and lots of hand sanding to get the curve right.
Bottom line is that you will become an expert on fiberglass work. While there aren't a lot of pieces made of glass, some take a LOT of time. Far more than I expected. But, done right, they can look beautiful and can have curves and fit that you can never get from metal.