As Red said, it can vary a whole lot. Really I would say max, maybe 50-60 minutes, as long as your REM cycle is. (I would sure love one like that). Probably you'll start off, if you're a begginer with perhaps 2-3 minute lucid dreams, but after a while, you'll find out how to stay calm, ground yourself in the dream, perhaps if you wake, re-enter a few times, and make it last a substantial amount of time. Just by relaxing, engaging,(feeling, touching) and enjoying in your next lucid, you should be able to really extend the time.

I wish you lots of luck...and perseverence.