Annoying Biomedical Electrical Engineering Question - How do you relate circuit values to find the distance of the nodes of ranvier?
Rm is INFINITE, not 0, except at the nodes. 

Draw the circuit from 1 node of Ranvier, where the current goes through the intracellular fluid, exits the next node, and then returns through the extracellular fluid to the original node.  This then becomes a simple voltage division question.  The longest value for which the intracellular voltage at node 2 reaches -45mV is your answer.

There will be some rise time at the nodes, because of the capacitance, but I think I would ignore it, at least as a first step.