Also, in people's experiences, how long before weather reports change at cruising altitudes? (i.e. can I leave it a day or so before flying and still have a *fair* representation of the likely weather, or is it so changeable up there that you could never tell one hour to the next?)
The weather at cruising altitudes is generally forecast (I presume you mean forecast rather than report) based on models initiated 24 hours before the valid time. E.g. the forecast for 1200 Tuesday will be based on a model run initiated 1200 Monday and will usually be available by 1800 Monday. It isn't usually updated. The output is usually graphical -- a significant weather chart indicating areas of CBs, CAT and jetstreams.
The airport weather is usually relevant only at the very beginning and end of a flight. It is forecast up to 30 hours in advance for major airports.