Most movies are fixed in a tangeble medium months before week one. Sometimes years. Component parts (principle filming, scripts, original music, if any) are routinely finished at least a year before release. A rush to get everything out to enjoy the full 3 years of income potential would further push out the couple of films a year made where people actually cared to do things right.

20-30 years might be right, though. Or one of the escalating fees for renewal after a reasonable period, preferably with a required deposit of the work so when it becomes public domain it can be easily distributed.