Dreams occur mainly during the phase of REM sleep in which brain activity is more intense. Dreams can also occur during other phases of sleep. However, these dreams tend to be much less vivid and memorable.
The length of a dream can vary, in fact, it can last from a few seconds to about 20-30 minutes. People are more likely to remember their dreams if they are awakened during the REM sleep phase.
The average person can have three to five dreams per night, but some may have up to seven in one night. Dreams tend to last longer as the night progresses and during a night of eight hours of sleep, the totality of the dreams can last from one to two hours.