How much rust is too much in the fuel tank?

Assuming it's not rusting through, ie the rust has affected the tank structurally, you can just chuck some nuts and bolts and screws in the tank, and shake it or set up a jig to keep it tumbling to remove loose rust. Then buy a fuel tank sealer, which is basically an epoxy coating which you pour in the tank and move it around and pour out the excess, and it creates new coating inside the tank.