Once created I start playing
I get an idea for a new league, I start rolling it around in my head for a day or 2, then start jotting things down on paper. After another couple days I pretty much have it planned out.
Open OOTP, create (takes a forenoon), fine tune and run a few tests (afternoon), usually start playing that evening.
So about a 4-5 days, but have done the entire thing in a day or 2 as well as taken longer. League size has a big bearing on that.
One thing I like to do during creation is come up with story lines for the league. Get a back story of course, but I like to create my own too as well as what's in OOTP.
One time I took a guy who was the #2 over all pick at 19 yrs old. He bombed out and was out of the game in less than a year. My storyline went like this.
20 yr old kid - dejected, depressed, not sure about life and future. Bums around for a while, job to job, but no real direction in life.
Heads to South America and works in a missionary for a couple years, trying to figure out life and what he wants to do. He finds faith and direction while he's there.
Of course all this time at the mission he also teaches kids how to play baseball and have fun. After a couple of years he really "finds" himself and feels a new drive in life.
People convince him to try baseball again, so he goes back home and signs with an Indy Lg team. Starts to do good and turn some heads. Next thing he's got ML teams knocking on his door. He goes on to a long career in the majors.
Anyway, you get the picture - nice little story about a kid who went from being a top pick, to the bottom of the barrel, and back to the top again.
That's the sort of detail I like to spend time on, but creating those story lines is always going on in my head, before, during, and after creation.