If you wipe down your strings before and after playing, you will get a lot of the grime and finger grease off and they'll last longer, as Cindy suggests. I find that as soon as strings start to feel at all hard or brittle I change them. New strings are much softer on the fingers. They also sound better, but since the sound deterioration is so slow, I never notice that my strings don't sound so hot until I change them and I realize how much better the new strings sound.
Since I like the feel, sound, and look of new strings, I think of it as a treat rather than a chore to change strings.
Ellen, if you wait until the cues that your strings need changing, you've probably waited too long. If you've waited a year, I bet you'll find new strings just feel much softer on your fingers and you'll want to start changing them more frequently. Professional musicians change strings at least every 2-3 months, as Guy does. I have a friend who plays gypsy jazz guitar in the SF Bay Area and has several regular gigs. One is the first Thursday of every month, and he changes strings before that gig, so he changes strings religiously once a month. I have more instruments than I play regularly, but on the dulcimers that I play regularly and on my six string guitar, I try to change strings every three months. The others I leave until I'm going to be playing one of them for a spell.
If you get the right tools, changing strings is easier. Make sure you have a string winder, a tuner, and a wire cutter handy.