What is the effect of having an excessively long dry motor period?

Having an excessively long dry motor period primarily leads to excessive starter heating. When the starter is engaged for a prolonged duration without providing fuel and ignition to the engine, it generates heat due to its continuous operation. Starters are designed to function within specific time limits, and exceeding these limits can result in overheating, which could potentially lead to starter failure or damage.

While other factors like cabin temperature might be indirectly affected by an engine that's not starting (in terms of overall aircraft systems), the immediate and direct consequence of an extended dry motor period is indeed the excess heat generated in the starter. This makes option B the appropriate choice in this context.