"Base stats" are *not* the numbers you see on your Pokemon's summary screen. Base stats are one of several factors that go into calculating the numbers on your Pokemon's summary screen (which you might call its "real" stats). I have illustrated this using a screenshot of Showdown's teambuilder, below:

Base stats are static numbers. They do not change when your Pokemon's level changes. Instead, they are passed into a formula that calculates your Pokemon's "real" stats. **The term "BST" simply refers to the sum of a species' base stats.** So, for Nidoking in the image above, 81+102+77+85+75+85=505. BST can be used as a rough gauge for how good a species' stats are overall. BST is not used in any calculations.

A Pokemon does not ever "reach" its base stat total -- it *always* has its base stat total, because base stats don't ever change. Base stats are used by the stat calculation formula to produce the "real" stat total. EVs, natures and IVs (which you can see in the picture above) are also used by this formula, and the Pokemon's level helps scale these numbers properly.

There is no point in knowing when Victini will reach 100 "real" stat points in Attack, because base stats are not the same as the numbers you see on its summary screen. You aren't supposed to think of it like that.

(Aside: Pokemon games have unfortunately begun referring to EVs as "base stats", even though fans have used different names for decades. Try to ignore this, the fan terminology has well and truly come out on top.)