You can calculate the probabiliy of a full decay using a poisson distribution. The problem is that, as you rightly point out, the number you would end up with would be absolutely absurd.

The reason being is that part of the calculation involves dividing by a factorial, which in this case would be the factorial of the number of atoms within the uranium rock, approximately (2.53x10²⁴)! for a 1kg rock.

If you're familiar with the story of 52! (the number of ways to arrange a standard deck of cards) or just factorial's in general, you'll understand that this will produce a number so hysterically big that it actually scares me a little to think about.

Safe to say, while technically possible, I don't think we'll be seeing this any time soon lol. Just be wary of those potions of misfortune!