Regardless of the route you take, having any "quality" images in your game will take a lot of time.
For hand-drawn art, it's the time to do the drawings.
For 3d renders, the time to set scenes/lighting/poses/render/post-work.
For AI image generation, the time to repeatedly refine your prompts and settings, and then editing/inpainting the results to fix the inevitable errors.
One of the big differences between them is how much time it takes to "get good". Longest would be learning to actually draw. Shortest would be learning the AI gen tools. But also note that doing AI gen does have a cost - you need quite powerful GPU and system.
The other thing to consider is how important "quality" images are to your project. Is it better to never create a game because you can't afford to create/buy the art you want? Or is it better to make a project with art that is not really as "good" as you want?
What do you like about games? Is is story? is it specific fetishes? is it the style of game (stat-grinder, vs VN, vs platformer with game-over-rape scenes)? Would a game with good story and the fetish you like still be considered "a favourite" if it had poor images?
Not every game type is appealing to every person. Clearly there are lots of people who like beautiful graphics but shallow, shitty, stories because they sell quite well on steam. But for me that sort of game is like low-quality McDonalds food - unsatisfying.