About 10-15 times a year I would estimate. I live in the country and have livestock so I have to investigate noises or disturbances. Most "bumps" are not going to be two legged, more likely 4 legged looking to make off with some eggs or chickens. I have indoor and outdoor dogs, and they often alert me of "dangerous" situations like the UPS guys is here or my wife is coming home from the grocery store.
My cat often alerts me at night more than the dogs do, nothing will freak you out more than late at night and you roll over and you see the cat frozen and staring off down the dark hallway intently.
Ironically, the only serious bump in the night that I have ever had happen, the dogs did nothing, my cows woke me up. I woke up at 3:30 am, couldn't figure out why. Then I heard the cows mooing incessantly. That set my spidey sense off so I jumped up, grabbed the Kimber off the night stand and peered out the window. I looked out and saw the farmers field across the road was on fire, not there was a fire, I mean it was ON FIRE, the whole frickin' thing! It was apocalyptic. Very surreal thing for a sleepy brain to comprehend.
It took me a second to notice that there was a truck sitting at the end of my driveway, about 100 yards away. By the time I threw some clothes on and grabbed the mag light, the truck had sped off. I ran down to the end of the driveway and saw two individuals walking along my road towards me. In the middle of the night, on a dark gravel, country road, with a raging wild fire mere yards away and they are out for a stroll...yeah right. I lit them up with the mag light as they approached and they stopped. One of them was holding his stomach and said "I really need to take a dump, can I use your bathroom?" That was probably the last thing I expected him to say, but the answer was easy. "NO!" Just then I saw flashing lights as a state trooper came tearing down the road. I decided he could handle these two clowns and when he pulled up I told him I was going back to my house to wake up the rest of my family in case the fire jumped the road and we needed to evacuate. He concurred, I told I didn't know who the two late night strollers were and about the truck that had disappeared and left him to deal with them as more emergency vehicles began arriving. Overall, a pretty exciting night!