Depends on too many things.When I was an undergrad in my education department, I often heard, "Teaching is an artform. It'll take you 3 years to get into the swing of it."
I worked taught in programs at schools and community centers for 2 years - 1 year as an undergrad and 1 year after. I've been doing this job in Korea for a while now, too.
Generally, an hour lesson plan takes me about 20-30 minutes to conceive and plan out - if it's original material or not. Afterwards, some time is thrown in for reviewing and revisions. Then I have to make materials which can range from another 20 minutes to 1 full hour.
If it's a lesson from our text book then the whole ordeal takes me 15 minutes - since I've more than likely done the lesson before, have old resources, or have done something VERY similar.
The more experience you have, the quicker and easier it will be to come up with and execute lesson plans.
