Response times : Licensing models / price offers.

The thing that can give you a clue for what to expect from a particular seller is to check their profile page (https://www.cgtrader.com/designers/ArtistNameHere) - below 'Rating' and 'Reputation' there is Response Rate and Response Time.

Thank you. Thats what I feared that was what it could be.

"How swift should I expect a reply to my messages?"

Between few minutes and never. The sellers here each have their individual lives and businesses. They are under no obligation to provide any additional service to the customers, be it for free or for fee. For many, selling stock models is just a way to earn side money and they are not interested in wasting time on fulfilling customers request. On the other hand there are plenty of sellers who will go out of their way to help the customer as best as it's reasonably possible. Don't be surprised if you'll end up at either end of the stick.