The rating of the breaker say 15 amps , can I pull a load of 15 amps continues, or is the breaker derated by 20%

If a load runs 3 hours or more it is considered as a continuous load, and the entire circuit, including the circuit breaker is to be loaded no more than 80 % of the circuit rating. So, a 15 amp circuit should have no more than 1440 watts running continuously.