I’ve read that a large asteroid could be positioned at a particular Mars L point and it would protect the planets atmosphere from being stripped by the sun.

I've read something similar, but I believe it has to be an asteroid with a magnetic field, whether permanent or electromagnet.

The estimate I heard is it would take 100,000 years for the atmosphere to be stripped off. That’s a long long time.

The atmosphere will be stripped away over a time period of millions of years though.