我以前研究过24小时说法的来历，首先这不是法律规定。下载盗版软件和电影，在1小时内删除和24小时内删除没有区别，都不能作为免责理由。

Is there a 24-hour rule under the copyright law that allows a user to download a program and use it for 24 hours to determine if they want to continue using it - and then after the 24-hour period they have to delete it or buy it?

No. The 24-hour rule is an urban legend created and circulated to justify violations of international copyright law on the Internet. Infringement may occur by downloading software, uploading software, making software available for download, and transmitting software files without a valid license to do so. Whenever the distribution of a computer program occurs without the consent of the software manufacturer, it is a direct infringement of international copyright laws. The unauthorized reproduction or distribution of a program, even for only 24 hours, is illegal. The amount of time is not an exempting factor.

出处：ACDSEE Piracy FAQ http://nl.acdsee.com/support/piracyfaq

不过我注意到国外律师事务所给网站发函要求删除侵权链接时（“notice-take down procedure”），一般都要求在24小时内完成；而很多大型网站比如Google也承诺在收到可靠举报后在24小时内删除侵权内容。根据美国《数字千禧年版权法》（DMCA）中的“避风港原则”，只要网站在接到权利人的通知后，及时删除侵权内容，就不用承担侵权责任。

所以“24小时删除”的说法可能和通知删除侵权链接的律师函有关，但和网站不同，盗版软件和电影的下载者不受“避风港原则”保护，所以24小时内删除也不能免责。而该免责声明也没有法律效力。