How often to retrain the time series forecasting model?

I have a weekly time series model, with 8 weeks as the forecasting horizon. There are 4 exogenous variables, which I forecast also 8 weeks ahead and use as the input for the target variable forecast.

Now I run this model every Monday. My teammate, without much time series knowledge, suggests that the model does not need to be retrained every week, and just train every 4 weeks or so.

I understand the models like classification does not need to …

datascience every exogenous forecast forecasting horizon knowledge monday series time series time series forecasting variables