How long does it take to make a work?
There is no average, there is no rule. It depends on where you want to start counting time and whether you want to consider the mental idea that precedes the physical realization.
Each work takes a lifetime.
I might make a red spot in the center of a giant blank canvas 20 years from now and have thought about it today, I might have randomly associated two colors on my return home forward yesterday and have time to put them together on canvas the day after tomorrow. I might have an unconscious desire to use yellow and never use it.
In concrete terms, the amount of time I am at work physically on the work varies depending on the characteristics of the work itself, the size of course matters as the intention of the moments I approach it. A couple months? Days or maybe just hours?
I could take a blank canvas and throw a sketch of black paint on it and feel already satisfied with the act and the form, I could be finished in as little as a few moments.
I might take a canvas and hold it next to me for months and change it and redo it a thousand and five hundred times until I decide or don't decide what it will look like.
I might have a definite idea in my head and pursue it linearly or I might never have any and let it go.
I might, and I always do, it's a time you don't feel but that leaves marks permanently.