there's no exact amount that you need to write, but there is a smart way of "writing", and that is....
1) target low competition keywords. These are keywords with zero, or very little monthly searches that don't appear in the title of the websites ranking on first page of google, and there's no authority site on the 1st page, either. Basically, if you find keywords that don't show up in the titles of the websites ranking on 1st page of google, and if those pages also happen to be websites like pinterest, quora, forums, yahoo answers, or unoptimized (mom-and-pop type of) websites in general... you have struck gold. Sure, the traffic you might get from such keywords might be very low (anywhere from 10 to 100 visitors a month) but if you target 5-6 such keywords in each article (which by the way, needs NOT be 4k words long) and you write 1 article per week for 1 year.... your traffic and sales will skyrocket.
2) write 1-2 articles per week for all eternity... or until you get bored, but don't get bored before 1 year passes... or until you make enough money to outsource the writing to someone else, whichever of these 2 comes first
If you do these 2 things properly you don't need 4k words articles (like holy crap, who is going to read all that??), and you also won't get burnt out within a week.