[SyneRyder]

Depends how you do it. Many apps that use time trials will often leave behind some preference file or registry key to block re-installing.

But it's probably not worth building that into your software (I don't) - customers will either get so sick of reinstalling every 30 days that they just buy your software, or they're so poor that they were never really going to be a customer anyway, so you may as well let them continue with the workaround until they can afford it.

Time trials can be useful though. While most customers buy within the first day of use (and typically the first hour!), anyone who tracks metrics knows that expiring the trial version also generates a boost in sales around day 29 / day 30 of the trial.