Depending on how the starter is wound, it will either reach a set maximum speed, or the RPM will increase without limit. Running a series-wound starter motor for more than about 5 seconds is inviting disaster- - - -as the speed increases, it can throw the conductors out of the slots in the armature. A shunt-wound motor will hit a moderately fast, but safe top speed and level out.
As a matter of precaution, I will not run a starter "no load" for more than a couple of seconds. I'll crank an engine with the spark plugs removed for considerably longer, but more than 15 seconds that way makes me uncomfortable.
Jerry
