I am thinking this is why my lucid dreams are not too long. Something will happen during the dream that I will associate with being the end of the dream.

Take for example, a lucid dream I had this morning: I caught a false awakening, flew out the window of my second story bedroom to fly and saw a nice sunrise. My body felt heavy flying down to my backyard. I saw a DC, mentioned the sunrise to him and asked him why my body felt heavy. He said it was because I was thinking too slowly. He gave me a phone number so I could call him (I forgot the number). Then he asked me to put down my signature on a tablet of his. I put down my signature, thinking explicitly I had plenty of time. Immediately after I finished my signature, I woke up.

I figure I was subconsciously associating the end of my interaction with the DC with being the end of the dream.

My dreams will end similarly after attempts at dream control. I will make an attempt at dream control and will, I assume, subconsciously associate the end of the dream control attempt as being the end of the dream.

I am thinking a way to do this is to stay one step ahead of myself during the dream. In my LD last night, I never thought about what I was going to do after my interaction with the DC. Perhaps the dream could have lasted longer if I thought to myself exactly what I was going to do next rather than simply thinking I had plenty of time.

This is the longest lucid dream I have had that I can recall at the moment. It was probably about 5-7 minutes in length. I'll make bold the parts where I was thinking what I was going to do in the future during the dream.
Spoiler for "long" LD lol: