The short answer is, it's different for every person and it's different for every dream. For example, I have a much longer lucid if there is a false awakening right at the start of the dream and I become lucid from it compared to another dream where I RC in the middle and become lucid.

Also it depends on how long you've been sleeping. Dreams are short early in the night, and get longer as the night goes on. For me, the length of dreams peaks around 8 hours and then starts to go down a bit after that as I've slept too long and have trouble staying asleep longer. There are a lot of factors like that which can affect how long the lucid will be.