How Far Apart Are Nodes Of Ranvier? I Have Been

How far apart are nodes of Ranvier? I have been searching for this info and found only one source that said about 1-2 mm. Is that right? I am wondering because if a signal can jump from node to node along the length of the axon, why can't it jump the 20 nm or so at the synapse?? I think it must have something to do with the concentration of sodium ions in the postsynaptic cell but I can't figure out how to explain/understand it for sure.