"Each agent was evaluated every 250,000 training frames for 135,000 validation frames" What does this sentences stands for? in DQN nature paper?
0: After 250,000 steps in the training loop they start a validation environment to check the current performance without fitting the weights of the current neural networks.

2: One episode of a validation episode just takes at most 135,00 frames (steps) and were not truncated by time.

1: Some games are more difficult to learn than others and the DQN approach is more suitable to some games than to others.