There isn't a standard way that software writers do this, so a blanket "do this to reset demo time limit" procedure won't work.
Some vendors store demo time limits in preference files, some in Application Support files, and some actually store hidden, binary files in a specific folder (and sometimes within the application bundle itself).
We would have to take this on a case-by-case basis: the procedure for resetting the demo time limit on, say, Bejeweled may be much different from resetting the demo time limit on, say, World of Warcraft (just two examples).