It is a lot of work. Don't think of it as anything but retailing, plain and simple. If you go into it as "a hobby" you won't have enough time to devote to the advertising and press releases and coding and content development, etc, etc, etc, you'll need to make a buck.
I know a couple people who make "a living" off CP, but that only means $35,000 a year or so, after years of development and promotion. I use CP to sell swag related to my comic strip, and lots of political bumper stickers and t-shirts. I have hundreds of products, and my store has been featured in articles in the LA Times, CBS Marketwatch, Detroit Free Press, and half a dozen other places, and stuff like that generates a lot of sales. But you need an angle, or else you're just going to get lost in a sea of mediocrity. CafePress products aren't cheap, so your designs alone will have to carry the weight. People can buy crappy t-shirts with Sponge Bob on 'em for $4 at Wal Mart, so what will be special about your designs to make people fork over $16-$18? It's not going to be the basic pocket-less white shirts CP offers. You need to have something clever and timely that they cannot purchase anywhere else.
I don't mean to discourage you, but a CP store is not going to bring you much money in the short term. You'll be spending so much time just coming up with the designs for your products and store, you won't have time to promote. Count on a year or so, realistically, before you can count on seeing any steady income from CP, IF you do all the rest of the promotion stuff.
My CP store is here if you wanna see a "Premium Store" in action...Good luck! And be sure to check out the CafePress message boards. A great resource which will save you TONS of time and angst.