I might go for 30 years: roughly a generation (of humans). If you wanted a different rule for software then you'd have to carefully define what is meant by "so

I might go for 30 years: roughly a generation (of humans). If you wanted a different rule for software then you'd have to carefully define what is meant by "software" and think carefully about side-effects: would you be discouraging people from releasing source code, for example? Perhaps not worth it.