A discussion (queston) came up at the field among us "electric whiz kids"!

None of us had a good answer so I am asking here.
A guy has a 6 volt battery for his receiver....... NO regulator... Is this bad?? He sometimes, after charge has 7 volts etc. Also.. he ask.... How long can I safely fly on this battery? I told him 4.8 volts, BUT I told him not to take my word till we know from "knowledgeble people". I am just using the logic that the receiver does not know what kind of battery is on the other end of the wires. It is designed to run on 4.8 to 6 volts. Is that correct...?? OR?? thanks... lownslo
Bob