How much rust is too much?

The problem with rust is there is only one good way to fix it. Someone mentioned rivets and bondo, yada yada - don't waste your time. New metal welded in, properly sealed and painted is the ONLY way it will not return quickly. Depending on where it's at around windows, cowl, etc, a complete strip down and rotisserie approach along with the aforementioned proper welding, sealing and painting is the only way rust repair ends up not being a waste of time. Considering the car's restored value, in most cases it is also going to be a waste of money.