Hi,
You probably won't be able to set up any hardware timer to interrupt at a rate as low once every 30 minutes - if you don't run the PIC REALLY slow that is.
What you usually do is set it up to interrupt at something like 10Hz or whatever and then keep track of the number of interrupts in order to get a total of 30 minutes (18000 interrupts in this case). That way you can use the same timer and interrupt for the LED flashing and any other timing tasks you might need.

Darrels DT-Ints are a great way to this.

/Henrik.