Because I am often bored while sitting on a plane, I pay attention to the most mundane of things. On my UA flights I've determined that (usually):
1.) The seat belt light goes on approximately 20 minutes prior to landing.
2.) The "Flight Attendants prepare for landing" announcement comes at approximately 10 minutes prior to landing.
3.) Sometimes this varies slightly and my theory is that the announcements are based more on aircraft altitude than any exact timing.

I even had one flight into DCA where the pilot forgot the second announcement and the plane landed with FA's standing in the aisle. They were not particularly please with the flight deck crew.

I also am pretty sure I know the video safety briefing by heart now too.