Take a look at File > Examples > 02.Digital > BlinkWithoutDelay and the associated tutorial:
You can set the interval by changing line 42 of the sketch. Remember that's milliseconds so you need to convert minutes to milliseconds (by multiplying minutes by 60 * 1000).
I recommend doing your initial experimentation and testing with an interval shorter than 45 minutes so you don't need to wait forever to see whether it worked.