100 years. Covers the lifespan of the author plus wiggle room.

If you make something unique, you should be able to reap the rewards of doing so for at least the duration of your lifetime. When you think about things that were created 100 years ago, it's clear that a century is enough time for things to have their moment in culture and also to be capitalized on.

25 years isn't even enough to cover music that bands recorded in the late 90s, so that seems very unreasonable to me.