I’ve heard people say that LD 's last only a couple seconds. (same with dreams, except they seem like longer) I was wondering- do LD’s seem longer, or do they actually seem that short? I’ve always wondered this 
Alot of LD’s do last only a few seconds, but only because people get excited and wake up. There are people that lucid dream for extensive periods of time but I’m not one of em so you’ll just have to trust me until one of them show up. 
depends onn how lucid youre are… and how lucid you stay. Try examining details of things, feel them and look at them… looking at your hands helps too. also see Extending Time
actually with me whenever I realize i’m dreaming, i think about it for an instant, then sometimes I go and do something else and lose my lucidity or i try and do what i pretty much always do, which is fly. but usually most of my dreams are vivid and i control them somewhat, but i take everything for what it is and don’t question it, and therefore don’t become lucid. i usually become lucid out of spontaneity, not RCs, but that’s just me 
so i guess some of my LDs are only a few seconds, but i don’t wake up, then i think i had two others in which i actually stayed lucid and did other stuff, but only for a few “minutes”
ND’s normally also last much longer then a couple of seconds, I dont know who told you otherwise but they aren’t right. We have like 100 or more minutes of REM a night in which we dream, during which ofcourse the dreamscene will change from time to time, but some individual dreams last for several minutes or more.
Experiments where subjects were instructed to count seconds showed that time in the lucid dream is the same as the time in the physical world. My lucid dreams usually last 1 to 3 minutes, by on occasions they last only a few seconds. I’ve had a few lucid dreams where I felt that they lasted more then 5 minutes. Some people say they can go beyond 20 minutes in a lucid dream.
My LD’s usually last between 10 minutes and half an hour. Though they will occasionally last less time (especially if I haven’t had an LD in a while).