From what I can tell on the Turkish Airlines site, free checked baggage allowances are:

International flights:
business class - 30 kg
comfort class - 20 kg
economy class - 20 kg

Domestic:
business class - 30 kg
economy class - 20 kg
promo class - 15 kg

Exception to this is if your Turkish Airlines flight originates through a partner airline...then if the partner airline has larger allowances, then those other baggage limits apply instead.

Their site shows that excess baggage fees vary based on whether it is a domestic (within Turkey) flight or international flight.