I have never done a bike trip shorter than five days, so can't comment on the really short ones. But generally, I trust weather forecasts out for about six or seven days. I always bring rain gear, even if the forecast says I will not need it. But otherwise on a short trip I would rely on the forecast for temperatures. But I will carry more clothing for a wider range of temperatures on a trip longer than a week.

I usually start out with three or four days of food if I know that there will be lots of opportunities to buy food. But otherwise, I might carry more. One trip I started out with over two weeks of food, on that trip I did go past a grocery store on day eight, that store was not on my map so I did learn that I carried a bit too much weight for the first week.

A trip longer than about a week and a half, I might plan to buy camp stove fuel somewhere on the trip, but a shorter trip I will bring what I need from home.

I do not like to run out of expendable supplies, for a longer trip I will bring more shampoo, a bigger bar of soap, a fuller tube of toothpaste, more camp soap, etc.

I usually do sink laundry every day or two, so I do not bring much clothing. But if I did a trip for less than five days, I would skip the sink laundry and just bring what I need.

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If you are new to touring, it might be a good plan to try to figure out a system for what is where in your panniers. For example, on my bike I can remove the front right pannier and the bike will stay balanced on the kickstand. So, the front right pannier is the first one to come off of the bike, thus it has my tent, that is the first thing I want in a new campsite. My tent is often damp in the morning when I pack it, so everything else in that pannier is stuff that can get damp, like flip flops, etc. My front left pannier, some trips it has had all my clothing, some trips it has had all my cooking supplies (pots, stove, fuel, etc.). If I carry a spare tire it goes in the very bottom of a rear pannier as I probably will never need to get it out. Same with my tools and spares, they are heavy and I do not expect to use them so they are in the bottom of a rear pannier. That said, I do keep a small multi-tool handy along with a easy to get at inner tube. Otherwise, for my rear panniers I have no consistent plan but at times I wished I did.