One of my goals is to visit every country. I've been to about 70 and have been to a few difficult ones (Turkmenistan, DR Congo, North Korea, Sudan). I figure the rest will take perhaps 10-15 years.
The expensive ones are typically islands and much of Africa is "roughing it" quite a bit.
I count a country when I have done something meaningful and gone through immigration (so the DMZ does not count as North Korea). Length of time doesn't matter too much, but typically 24 hours minimum.