The longest they normally can last is about 30 minutes. Sometimes dreams become unstable though if you go against the flow of the dream and cause you to wake up.
Try this: When in a dream do not focus on anything that has to do with the waking world. It is very tempting to compare the two while in a dream. Don't think about waking life. Do not think about how you are actually sleeping in bed. Just stay aware of which reality you are currently in, the dream reality.
The dream reality is very real. Even if you do not remember your dreams the emotion of the events that happened in your dreams carries into waking life.