How long should the Nikon D3100 last.year? Months?

I bought the nikon d3100 about a month and a half ago

At least nine or ten years. I have a Nikon D100 which was purchased in 2002 and it is still performing perfectly.

Just take care of it as you would any fully adjustable camera.

I have 35 mm cameras that are over 50 years old and they still work just fine (Nikon F2 and Leica M2)

I agree with David. Properly taken care of, stored in a bag or case all the time it's not in use. Keep your gear clean and free of dust - as much as you can. I always pull the batteries from my cameras when they're in storage. You should get a 100, 000 shutter count at least. For the normal amateur that will take many years.

If you take good care of it then no reason why it shouldn't last 5-10 years and beyond depending on how much you use it. It has mechanical parts that will not last for ever
My EOS450D is 30 months old, 20, 000 actuations and still works as new; my son has a 5 year old, 50, 000 actuation EOS400D that is looking well used on the outside but still produces fantastic images. He is, however, beginning to suspect that the shutter isn't quite accurate and is trying to decide whether to repair or replace.

If you take good care of your camera, it is possible for it to outlive you.

Indeed, when used and cared for correctly, or even over-zealously, a DSLR cameras can last the better part of a decade, if not more.

It all depends on how you take care of it. I have a D70 that I bought in 2004 that my son uses. It is still going strong.

If you take care of it, it will be obsolete long before it will wear out.

Before digital, I used a 1976 Pentax KX, it still works perfectly.

My 1980s Fujica AX-5 (the oldest electronic camera I have) is still going strong.

Look after your kit and this will be the least of your worries.