More than the authors life is excessive.
I'd prefer lifetime plus some short additional period (say 10-20 years) just for safety's sake. If you make it just lifetime it wouldn't surprise me in the least if some bright, young corpling arranged for an accident or two to 'free up the rights issue we're having'. To a corp it may be considered an acceptable risk to put out a $100k contract rather than fork over a few mill for the movie rights. Just sayin'
skribe
The reason that TV lost is because people choose to be actively involved in how they spend their entertainment and downtime, rather than being spoon-fed what someone else wants you to watch. Piracy is popular because while people like the shows they want to watch them how and when they want them (sans interruptions like ads). Gaming is popular because you're the hero rather than watching some overpaid action doll doing all the fun stuff. TV is passive. The internet is active. Come get some!
Where there's a will, there's a relative.