It's probably on an artist by artist basis.
The length of planned script is very dependent on the story, the author, etc. Some might plan only a few weeks ahead, and do a little foreshadowing and use it as a base later, or some may plan up to 2 years ahead of time, plus the rough outline for an ending, and will be able to foreshadow easier because they already have an idea where the story is headed.
The actual process of drawing and publishing takes awhile. They don't send a chapter out the night it's to be printed, and then begin work on the next chapter. Each one is worked on for a period of time. How long? Depends on a lot of things. I honestly don't know. I'd guess maybe 2 months for early series, probably just a few weeks for older ones, especially as the artist gets more skilled and his assistants get better at drawing his style. Then again, I'm going by American standards. I have no idea what magic the Japanese use, but they can do some pretty impressive stuff with a low budget and small time frame, stuff any American would give up on in minutes.