Looks like the starter relay gets its power via the ignition switch, low rpm warning unit and radio relay. This may just be an ease of routing thing, where it's simpler to take power from one point to another rather than double up on wiring, or there may be other reasons - not sure. Power to the ignition switch itself will be interrupted if the rotor brake is on.
As far as starter limitations go, I'd be pretty confident it's much like your car - you could keep cranking as long as the battery held up and you wouldn't damage the starter motor. It's not like a turbine where it has to spin up to a gazillion rpm, and nor do you have to worry about hot starts.