It's not clear what type of application you're going to develop (desktop, web, console...)
The general answer, if you're developing Windows.Forms
application, is use of
System.Windows.Forms.Timer class. The benefit of this is that it runs on UI
thread, so it's simple just define it, subscribe to its Tick event and run your code on every 15 second.
If you do something else then windows forms (it's not clear from the question), you can choose System.Timers.Timer, but this one runs on other thread, so if you are going to act on some UI elements from the its Elapsed event, you have to manage it with "invoking" access.