This is my planning process - maybe it will help. Sorry it's a little long.
It takes me about 2-3 hours per subject to plan for the school year. This does not include all the research I do online. Once I know what books, notebooking pages, online resources, etc I plan on using then I can usually put it all together in a few hours per subject. I have 5 children but only homeschool 4 so I'm planning about 26 subjects in all so probably about 80 - 100 hours of actual making lessons. I have spent that much time or more on research.
I start research for the next year starting in January. This gives me plenty of time to know if what we are currently using is working for us and if any changes need to be made for the upcoming year. I research one subject at a time - seeing what's new, improved, etc - I take notes, website url's, prices, what I like/dislike and keep all this in my school binder for future reference.
As the school year ends (around may/June) I have all most subjects researched and the planning begins. I take about a month off (no school research or planning) then I go over my notes, do a quick research for each subject again (in case anything changed since my original research) and make a list of what I want to buy. Some subjects I already have what I plan on using so no need to research except for maybe some new supplements. Then I start buying.
Once I have all the books/items, I start planning it out for the year. I take my time and try to get at least one subject done every 2 days. I planned music and art study in the same day in about 3 hours but some subjects like math or english take me that long per child. I usually need about 6 weeks to plan lessons for a year.
If time is not on my side (is it ever? ;-)) I only plan for half the year so this may be an option for you. If you think everything will be calmed down more in say 6 weeks then just plan to that time, take a week off from school or do light homeschooling and take that time to finish planning your year or at least planning for another 6 weeks. Remember, there are no rules saying you have to plan for the entire year at once.