By default, the Windows Media Center pauses Live TV for 30 minutes only. This limit was probably placed to restrict the usage of disk space. But you can if you wish, increase the size of this buffer to, up to 120 minutes.
MCE Live TV Buffer Manager
MCE Live TV Buffer Manager will allow you to increase the size of the Windows Media Center Live TV buffer.
The tool will allow you to change the size of the Live TV Pause Buffer from 30 minutes, up to two hours, in 15 minute increments.
This is especially useful if you have at times paused a live show and then returned after half hour, to find that it had already started replaying.
This freeware app works on Windows 7 Media Center too!
Download page: MCE Live TV Buffer Manager.