Exactly. There are usually two paths to go on this.
The first would be to time-box it, where you put some logic in your code that checks to see if too much time has passed and disables the app, or popup an annoying message or something to remind them to buy the full version.
The second would be to create a "lite" version which has limited functionality.
In my app CreataScene, I took the second approach. My trial version has less content available, and users cannot save their pictures.
An easy way to approach this is to put a flag in your code like FULL_VERSION = true, and have your code do your special logic that either limits or timeboxes based on the value of that. This way it is easy to create the trial or full version without having to do code changes or keep separate branches for each.
