The politicians have set a self-imposed limit[1] on how much they can use per year. It's currently 3% of the funds value, to ensure we don't need to reign in too much during hard times[2].
However politicians have tried to use less, for 2026 they plan[3] to use 452B NOK which is roughly 2.13%.
However as OP points out, the total budget is around 2100B NOK, so the oil money pays for roughly a quarter. And that's becoming a bit of a problem in my view as well.
[1]: https://en.wikipedia.org/wiki/The_budgetary_rule
[2]: https://www.regjeringen.no/no/tema/okonomi-og-budsjett/norsk...
[2]: https://www.regjeringen.no/no/statsbudsjett/2026/statsbudsje...