When I taught traffic school many years ago the rule was to leave one second following distance for each ten feet of vehicle length. (That was before the National Safety Council changed the recommended following distance from two to three seconds for cars) If your tow vehicle and trailer measure 50 feet you should leave (if you can) five seconds.
Start your count when the rear of the vehicle ahead passes a lane divider stripe. Count until that stripe disappears beneath your hood.
I know this is not always possible, but it is important for your own safety to try.
