That depends a lot on usage. Since there isn't much on the board which can leak, explode, probably the first issue it will face is data retention of the chips and decomposition of plastic.

If you are using it however, it will get dust, greasy, the flex cable will be worn, so obviously, lifespan will be shorter.

I am a mobile+PC collector and even some lucky laptops from 90's or even 80's still work, so it is very possible that an N900 survives 20-30 years if it hasn't been used much.

For this reason, I have one boxed in almost original condition.

Also I don't know of N900's randomly dying as LG G2s or Huawei Y550s, so probably they don't have a built-in HW fault.
Though the LCD in my original daily N900 has been replaced 2 times already, one time stuck pixels attacked, second time the chip which handles the signals (serial to parallel?) gave up. No image just heat. A lot of heat...