Leviathan.Kajii said:
It's been a while so I don't remember, but day 7 = 100% Nidhog?
The chance of Nidhogg spawning increases for every day (after Day 3) that he goes un-spawned. After 7, I believe it's a greater than 85% chance of Nidhogg. In theory, it could go on indefinitely without spawning, reaching a 99.99% chance after "X" number of days. That, of course, is very unlikely.
But, to answer your question more directly: no, it's never a 100% chance, ever.