My personal record for wearing out a chain and cassette is 3 months, but that was mountain biking in a UK winter... 
It is worth replacing the chain and cassette together - mix worn and new and you won't likely see significant improvement. The other thing you really need to look at for shifting is your cables. If these are pretty new, you probably just need to tighten them via the barrel adjuster (thingy where the cable joins the shifter and/or where the cable meets the derailleur, if you were wondering) - a one minute job worth trying even if they're kinda old.
Last edited by DirtDiva; 02-07-2009 at 02:26 PM.
Drink coffee and do stupid things faster with more energy.