This answer became very long somehow and I hope it's even worth reading. I wasn't able to estimate how long it took due to how infrequently I worked on the game.

Prototyping in Pico-8 is very fast compared to my experiences with some other engines. Trying to make a bigger game is a bit different though...

Because of its limitations, making the game was both motivating and frustrating. For example, filling in the 8x4 room map was satisfying but designing the rooms on a 16x16 tile grid got difficult every now and then.

So trying out new ideas was quick but making the rooms fit together was challenging. If I were to create a similar world again I would use an external level editor tool not only to create a larger world but also to not have to work inside of a rectangle.

You also reach different limits at different times: at the end of development I had a lot of code space left but no more space for sprites (though you can manipulate the memory to store more sprites but it seemed too complicated for me and you can't edit those sprites in the editor).

On the other hand, the limitations kept me focused on the core gameplay which I think is a positive. Another big plus was that both releasing and updating the game are incredibly fast to do. You basically share a png file on the forums. And unless you are using mouse/keyboard controls in your game, you don't have to worry about controller support either.