Sorry I'm quite the beginner in the field of NLP, as the title says what is the best interval for optimization in Mallet API? I was also wondering if it was dependent or related to the number of iterations/topics/corpus etc.
The optimization interval is the number of iterations between hyperparameter updates. Values between 20 and 50 seem to work well, but I haven't done any systematic tests. One possible failure mode to look out for is that too many optimization rounds could lead to instability, with the alpha hyperparameters going to zero.