On average, owners reply to messages within 4 hours. Keep in mind, you may be in different time zones, or the host may be without access to the Internet at the moment.
If the owner has not responded, please don’t be upset! Feel free to see other accommodation options in the area. You can message as many owners as you like and it is up to them to take a risk losing their chance at having you as a guest.
You can also check owner's trustworthiness on the page of the apartment in the section 'About Owner'