How fast can someone become seriously ill?

My step father's leg was being weird, he couldn't use it and all he could do was drag it so he thought it was a result of his back issues. He went to the dr and about a month later, he was gone. Cancer was everywhere.

Seconds, minutes, days, weeks, months..

Not sure what's going on with the case you are describing. It fits so many diagnoses.

That's so sad hope they get better 😞

Mercury does not stay in the body forever; it can be excreted over time, but the duration depends on the type of mercury and the level of exposure. For example, methylmercury (the kind they put in vaccines) can accumulate in the body and may take months to years to fully eliminate, while elemental mercury (photo below of this type) is cleared more quickly.

Yes that could happen. So many things it could be.☹

It can happen quickly.