They do not have to be short. I have had lucids that lasted an hour or two in real time, and some that lasted days or years in dream time. Generally, your longest REM periods happen towards the end of the night. If you add to that some REM rebound from lack of sleep the previous nights, you can get some amazingly long dreams.

It is tricky for me to keep track of time in any kind of dream, lucid or not. I get a lot of false memories that make the dream seem a lot longer. For example, I will start the dream with memories of an entire year, I will experience a 15 mnute scene realtime, then have memories of a day passing, and then I will experience another 30 minutes of real time dream.