They can last as long as your REM state lasts, which is very short at the beginning of the night, and gets longer and longer until close to morning, almost all of your sleep is REM sleep. At this point, I believe your REM stage lasts around 45 minutes. Could someone possibly verify this?
Also you should know that it is possible to make your LDs *seem* much longer than they actually are. I've heard stories about people who've had LDs that seemed like they lasted weeks, even months, even though they really weren't.
Happy dreaming