Not the only source but I couldn't bother to search more.
EDIT: this article says 90 years normally and 5 years if everything was run on nuclear power https://phys.org/news/2011-05-nuclear-power-world-energy.htm...
If nothing else, people havn't seriously looked. For example, in Australia, the state of NSW "proclaimed on 14 September 2012 ... [the removal of] the ban on uranium exploration" [0]. Australia has worlds most extensive uranium resources in the world, although not in the state of NSW for obvious reasons. Possibly we can extend our lead.
But practically, the 'resources' definition that that article is relying on (going purely off the Australian legal framework) likely involves investment on the part of companies to be able to advertise. We've a lead-silver mine in Australia that have '20 years of resources' on the books over 80 year periods, because every time they need a mine extension they go out and 'discover' a bit more of the resource that everyone knows is there but havn't officially measured yet. The current 20 years of resource is a polite lie as well, they've got lots more.
[0] https://en.wikipedia.org/wiki/Uranium_mining_in_Australia#Re...