I am a little confused here. 
Woke up at 4AM as usual, wrote down my dreams, went to the bathroom, and read about lucid dreaming some more. Turned off the light and went back to sleep at 5AM.
Woke up at 6:45. In this time-period I had a really long and cool LD, and a short dream beforehand with an obvious dream sign (being naked in public LOL) that made me realize I was dreaming. So how does one figure the length of the LD? 1 hour 45 minutes minus the time it takes to get to REM? That raises another question, what if you know from experience that sometimes it only take you 10 minutes to start full on dreaming?
It seems that the length of an LD is fairly hard to figure out. 
True, it’s always a subjective estimate, thereby adding the fact that time in LDs acts differently too makes it even harder to decide. I usually do it on gut feeling combined with certain times ( eg time I was awake, time I woke up, if available)
the only sure LD lengths are from lab studies where the subject would eye signal the time they become lucid.
I base my length by my gut and feelings 
it usually has right.
same here.
the best way is to measure it against real life. just guess how much time that all of what you did in the dream would take IWL.
some researchers believe as well that there is such a thing as “dreamtime”, which is different from real time. if this is true, then going by a clock won’t help. so i suggest, as others have, is to just go with your gut. how long do YOU think the dream was?
Honestly, I don’t trust my gut when it comes to guessing how long a dream was. I had dreams which had storylines covering hours or days! That being the mentioned dream time, obviously.
But maybe it’s different in an LD, I can’t tell, I didn’t have one yet…