Good points made. I definitely don't want to be one of those "bring the kitchen sink" type of people. Still, there are some things that I think are worth adding to my long tour packing list. For instance, it's pretty easy to plan for weather on a short tour in the pacific northwest summer, but a 2 month trip across a range of climates would seem to require carrying gear for various contingencies (rain jacket, arm/knee warmers, etc.). It also seems prudent to carry a few maintenance-related items that I wouldn't bring on a shorter trip (portable cassette tool, pliers, spare tires). In addition, I gather that the western portion of the ST has some significant low/no service sections, so having space for more food and water seems like a good idea.
There is a ton of variation on what all of that means to different folks. I have not done trips shorter that 9 or 10 days so I may not have the same perspective, but I always figured that you plan the tools and supplies that might be needed regardless of trip length. Clothing can be a bit more of an issue because on a multi month trip you may be in different climates.
That said I took pretty much the same stuff on the Southern Tier as I would on shorter trips. My total base gear weight was 14 pounds. That included bags, clothing, cooking and camping gear, but not bike, rack, water, food, or fuel. If you include the bike itself (24 pounds including bike, rack, tool/spare parts bag) the total weight for bike and all gear was 38 pounds plus food and water. I am probably pretty near the bottom end of the range of what people carry though.
At the other end of the range I met folks on the same route that probably had 150 pounds of gear.
Anywhere in that range is OK if it works for you, but I think leaning toward the lighter end probably makes sense.
BTW, another factor is that folks packing for an over-nighter or a few days probably don't feel the need to obsess over what they are carrying as much. They might also be more inclined to pack more fresh food from home. So in some cases they might actually pack heavier.
One plus about the low/no service areas in desert areas is that if you are in a bind most cars will stop and help. I didn't need to take advantage of that, but a guy I rode with some of the way had several major breakdowns and never waited more than 20 minutes for a ride when he needed to hitch.
As far was food and water on those places where you will be 80 or 100 miles between services... There are only a couple places like that. So my solution on the ST was t to have a 12 ounce 18 liter backpack (REI Flash 18) that I could put a few liters of water and maybe some food in. I used the water in the pack first and never needed to carry any substantial weight in it for very far. When I didn't have the pack I have crammed a couple recycled 32 ounce gatorade bottles in my jersey pockets. Of the two the pack was actually more comfortable and was great as a laundry bag, bear bag, grocery run bag, and so on.
In regards to departure time, I know that leaving from San Diego in late September will force us to contend with some hot weather in the early part of the trip. But it seems manageable and it fits best within our life schedule. We'll have to see how it goes, I guess.
I hope it all works out well for you. If you like hot weather it may not be a hardship. It would be miserable for me, but as i have said I hate hot weather for touring.
Have a great trip!