At what point does the equations of classical physics start crumbling?
What do you mean exactly? Can you specify? Do you mean, the boundary between the classical and the quantum case?
When having value of gravity as infinite?
At this level, gravity is irrelevant (fundamental particles are very light).
Yes where is the boundary between them and what defines that boundary?
What is important is to evaluate the magnitude of a typical action (the physics action) and compares it to the Planck constant. If the latter is negligible, a classical description would work.
Thanks for the answer :) I am going to look more deeply on the Planck constant and understand the consequences.
Apart from being the smallest quantity/measurement and using it in equations during college, I know nothing about it :)
it is used a lot in the equations behind many applications used in real life (I mean, beyond textbooks). But no one tells it ;)
I will read on this and get back to you in case of further questions :)