What should be the training frequency of a rnn model for timeseries prediction?
It depends on any number of factors. What kind of accuracy are you currently getting? How often are you getting new predictions from your model? Do you have human intervention at some point to check your errors, relabel, and improve your training sets? 

It's also possible that you don't need a schedule, per-se. Depending on your data, the pipeline and the programming of your model, you could also just have continuous learning built into your model with a new release of said model after every epoch. So, really, a lot of the answers you are seeking are very specific to your implementation.