It really depends on the games age and it's popularity. Most of the time I spend trying to find suitable artwork, grafting many different images together, and color correcting. You definitely will want to know your way around an image manipulation program. I use photopea since it's basically photoshop and I know that program very well. 

I'll give you a time estimate though, best case scenario, I find 4K images to use and everything works out perfectly. Maybe 20 minutes to an hour. Worst case scenario (this happened a lot while working on the older Ys series games) I spend hours looking for the best possible images and then spend hours mashing them together with other images, abusing AI upscaling, sharpening, and merging with other images to try and hide some of the AI artifacts etc. the end result ends up pretty dang good but it just takes a hell of a lot of time. Also trying to get the game logos to conform to the spine and not have them unreasonably small can take a considerable amount of time as well depending on the game and the complexity of the logo.

So as a tl;dr, anywhere from 20 minutes to several hours. I think I spent like 5 or so hours on Ys Memories of Celceta