That's not much - $25K per person over 10 years, and with an average income of $70K per person, and something like half of their economy is energy (most if not all social programs are funded from energy production).
If oil were to stop tomorrow, they'd have around 10 years before the money runs out. (Maybe a bit longer since kids don't earn money.) And if they have to replace all the social funding, they'd have even less time.
Norway is relying on slow reduction in demand, and time to transition their economy. Will they have it? It's far from certain. I feel like Norwegians rely too much on this fund to save themselves, rather than transitioning away from oil now.