So when it comes to writing and getting ideas for your writing projects, it's not a secret that coming up with ideas isn't usually easy. For most writers, coming up with ideas that are good enough and that have potential is a real struggle that takes a lot of hard work.
Indeed, even though writing the script is the most time consuming task in the whole process, first you need to be able come up with that premise for your script. That is something that requires effort for most and shouldn't be overlooked in the big picture.
So when it comes to getting those ideas, how long 'on average' are you supposed to be spending on getting those solid & executable ideas? How long does it usually take before you feel 'comfortable' enough get started and how much work is required from you?
Project-wise, we have basically three writing categories that need to be discussed: 1) ideas that are good enough for basic articles 2) executable ideas for thirty minute sitcoms / one hour dramas and 3) solid ideas for movie screenplays & books.
So the first and by far the easiest category is of course the article writing category. This is the category where it's easiest and fastest to get ideas that can be turned into articles (1000-2000 words) that your readers will hopefully enjoy reading.
In practice, if you're actively trying to come up with an idea, my experience (more than 400 hundred articles) is that it usually takes a day or two to come up with an idea. This is if I'm actively trying to find something creative to write about (I don't count reviews as ideas).
The second category is the one that has to deal with coming up episode ideas for television sitcoms and dramas (existing tv shows). This is where you're already going to spend considerably more time trying to come up with a solid premise to write on.
Based on my experiences as a tv spec writer (nine specs total), I've usually spent about a week actively trying to come up with an idea that has potential. It might easily take 40 hours of staring at a blank paper before something clicks, so you need be prepared for it.
The third and the most time consuming category is the movie script / book format. This is the category that isn't easily quantifiable and where the terms passive / active thinking come a lot more to play when you look at how long it takes to get things in order.
Based on my experience, if you're 'passively' getting ideas for a new project, you can spend a year before you get a solid premise. If you're not actively pursuing a new idea, it's going to take a lot more time to get started (bookmarking, casually writing down stuff etc.).
On the other hand, if you're actively coming up with a potential story arc, it's still not going to be easy on average. You're probably going to spend at least a month trying to do some initial world building, planning and structuring that could lead to something solid.
In that sense, when it comes to the question of how long it takes (and realistically should take) to come up with ideas for your projects, it really depends. It depends on the size of the project and how actively you're willing to work to get those ideas.
After all, first, if your task is a relatively simple one (article writing etc.) and you're willing to actively work to get those ideas, it might not take that long. In that case you might get an idea even the same day, so you'll get to the writing part of the project really quickly.
On the other hand, if you're thinking about starting a big project like a book and aren't actively planning to get ideas, that's when it's going to take a lot more time. In that case, it might take a year or more before you'll get an idea that is good enough and that has enough potential.