That depends on how they are kept. If you keep them in a bowl, you'll be lucky to get a year out of them. If you keep them in a proper sized tank, or better yet, a pond, you can expect lifespans well exceeding 10-20 years.
Well, I don't know how many gallons that is, but it doesn't sound like enough for two goldfish. They need a bare minimum of 20 gal for the first fish, and 10 gal for any additional fish, though personally with commons I feel it should be 20 gallons per fish due to their massive waste output. In other words, you need at least 30-40usg if you want your fish to live their full lifespan.
Remember, fish keeping shouldn't be about "well how long will they live in inadequate conditions." It should be about providing optimal care for healthy, happy, long-lived fish.
By the way, that banner in your signature is my own personal banner (as in, I made it). I'd appreciate it if you ask permission before using it. Normally, I'd ask that you remove it, but since I want the message spread, I guess I'll accept you using it since you seem to have linked it with your own photobucket account.
S'ok; like I said, I prefer if permission is asked, but you were good enough to put it on your own photobucket account, so I don't really mind.
Do you know how many gallons your tank is currently, and what size your goldfish are? So long as you keep them very clean, they should fair decently enough until you can get them a bigger tank.
Also, while goldfish have a projected lifespan of 20+ years, remember that it is just an estimate. Even people who take wonderful care of thier goldfish do occasionally suffer losses, as not all goldfish stock are healthy. And there is of course always the potential for accidents, latent health problems, etc. So if they die younger than that even in ideal conditions, don't beat yourself up too much; see if you can find the cause of the death, to avoid it happening again, and be happy that you at least gave them the best chance at a full, healthy lifespan.
For two goldfish, you should be looking at a tank that's about 30 x 12 x 18 inches. In a tank that size with regular 25-30% water changes, good filtration and a high quality diet your fish could well live for over 10 years.