From what I have seen...
Most people drive, regardless of part of the country. Get 2-3 paddlers in a crew and rotate driving and get out west in 2 days. Pros to this method is that you have a shuttle rig, all your gear, and stuff like stoves and coolers. Cons are it takes extra time and driving a really long way sucks.
Some people fly. Find an airport close to your destination: sacramento for central sierras, boise for idaho runs, SLC or Flagstaff for desert runs. Option A is to put all your gear on the plane. Major issue is getting a boat on the plane as only a few airlines will do it. Option B is to fly out with all gear except boat and/or paddle and rent a boat at the local paddle shop. Pros are you get in much quicker. Cons are it costs more, can be trouble bringing stoves and fuel on airplanes, and can be tough to get a boat on an airplane.
A hybrid option is to hook up with a crew that is driving, that you can fly in an meet somewhere. Send your gear with the road crew... fly in, rent a car and meet them up in the mountains and paddle.
As for time committment... most folks want to take at least a week for a trip that big to get enough paddling out of it. Some folks do a stealth 3-5 day fly in mission to get a specific run at a specific flow with a narrow time window.
Its easy to rent boats, and just about every modern creekboat is big enough to self support out of. You don't need an XP or "expedition" boat unless you are doing the grand canyon or something with mega flatwater. Even then, paddle shops should rent that type of gear.
Check out Earthen Exposure
for overnight self support info. That will school you up and get you on the right path.
If you had a specific run, state, or difficulty range you were looking at you might get some more detailed answers.