IMO it may depend on the gun. I know that if I had a 3 1/2 inch Registered Magnum I would be certifiable. I'd spend every waking moment that I could either looking at it or handling it with white cotton gloves.
You need to own a total of 250 firearms before you can apply for the certification. You then need 3 references who can vouch for your nuttiness and you have to pass a written exam. If you pass the test and the references check out, your county sheriff has 40 days to issue the certification. Make you you check that your certification is recognized by other states before claiming to be a gun nut in another state.
As with most things in life, just listen to the voices in your head. Sure, they may not be real, but that doesn't mean that the advice isn't valid... They'll likely tell you how many guns you need to have.
Failing that test, try to always have a number equal to your age. Thus if you were 31, you'd want to have at least 31 guns, etc.
I've been a gun nut since the time when I owned only a Lone Ranger cap pistol. I knew I'd eventually own real ones and that I'd be very happy to own them. I've been a gun nut since I was five.
By the time this picture was taken I was ten. I believe I was fully certified by then.
Depends on your political affiliation. If you are a Republican and own one gun grip, your Demo friend will call you a gun nut. If you are a Demo and refuse to own a gun because they are dirty evil things that are used ONLY to kiil innocent animals and wonderful missunderstood people who are tring to kill you, you are also a gun nut!
State of mind more than anything else. (A good indicator though is when you can have lengthy and mutually enjoyable conversations about guns with complete strangers either in person or on the internet.)