For me the planning is scaled differently for the size of the tour:
- For a quick overnight or two/three day trip, I am typically not very far from home. So I'll make my final decision the week preceding the weekend, finalize where I will stay each night, pick some routes and just go.
- For a trip of around a week, I might fly/travel elsewhere. So in this case, I'll make my rough plans a month or two in advance. I'll look through an approximate route, read journals of others that have traveled, find out candidate places to stay (e.g. parks or motelguide.com). I'll figure a rough mileage "budget" with several alternatives. For example, rather than finalize exactly areas to stay, I'll know town X is at mile 65 and town Y is at 85 and town Z is 110. If on a particular day I start at 0, I can wait to decide whether I reach X/Y/Z. I don't look up bike shops explicitly but sometimes that is something I look in Yellow pages along the way.
- For a trip of a half a year or more, I'll start my planning a few years in advance. I'll look up climates, visas and big parameters I need to get right. I'll make plans to wrap up my work/life/home and move things into storage. I'll have read journals and pages I can find and index them on my own web site I've created for the trip. I won't necessarily know where I am going day to day - but I will know rough time budgets involved.