Being more-responsive to models’ messages is better, especially if the model is from a different country and travelling to your Shoot City for the first time. Models are likely to be anxious, and unanswered messages from someone about to make a sexualised shoot of them adds to that anxiety.
However, Shoot Producers have busy lives, and are not expected to be responsive 24/7!
In general, a Shoot Producer should respond to a message from a model within a few hours, or a few minutes, even if it’s just to acknowledge the message and commit to getting back to the model by a certain time. For example,
Hey Jane, I am on a shoot right now, but I’ll get back to you by 8pm tonight.
Of course, the Shoot Producer must actually respond to the model by the committed time.
However, if the model has an urgent issue that the Shoot Producer is responsible for (for example, the model is at the accommodation, but they cannot get inside), then the Shoot Producer is expected to respond to the model helpfully and promptly.