For me, it seems to increase as my dream recall and general practice increases. My second to latest lucid was one that showed up completely out of the blue, after a long period of no practice, and yet, it was my longest lucid yet (about 5 minutes), and I even stabilized the dream for the first time, which contributed to the longer amount of time.
While I don't have much experience with it myself, stabilization does help if you do it right, and just keeping up to date on practicing techniques, writing in your dream journal every day, and staying interested seems to increase the length of lucids, even if it's only by a little.
Just my thoughts.