(Graph Convolutional Network (GCN) based recommender system maintenance issue

Question

I have built an item-item recommender model using (Graph Convolutional Network (GCN) for an E-commerce website. Could you please help me with the maintenance of the model.

1. How often should I retrain the model after deployment?( 1 month, 2 months)

2. What are the main metrics which I should track after deployment to evaluate the model?

3. Any suggestions related to Train/valid/test split ratios (such as 70:15:15 or 60:20:20)

4. Any materials related to model maintenance/recommder systems evaluation post deployment would be of great help.

Thank you