Extended engine motorings:
 

B737NG
• Starter usage is limited to 15 minutes for the first two extended
engine motorings. A minimum of 2 minutes is needed between
each attempt.
• For the third and subsequent extended engine motorings, starter
usage is limited to 5 minutes. A minimum of 10 minutes is
needed between each attempt.

Can anyone explain :confused:

Starter cycle duty times is due to heat issues. If you keep going past the limits (not sure why you need a 10 minute motor?) you stand a good risk of the starter having a catastrophic failure. :ok:

Then you have No Starter and have to start it by hand, like in the good old days. :rolleyes:

OVER ENGINEERED?
 

don't see any reason to motor for 15 minutes. the most is probably for say 5 min in the event of a tail pipe fire. haven't the faintest idea why one would require15 min twice and another 10 min on top !:rolleyes:

As per our Engineering manuals, the extended motoring is required after a wet start/tailpipe fire. In case, the 1 minute is not sufficient.

No experience with longer than a 5 minute motor, on start I recommend fuel and fire on after min motor at the time when acceleration starts to hang.

Would air start at altitude require a longer crank time?

You wouldn't have to use the starter motor at altitude. EICAS will generate a windmill airspeed necessary to restart. (I presume in the event of bleed failure)