Because our therapists are real people who have multiple clients, families, lives, etc. (just like you and me), each therapist is a little different.
Usually the therapist will respond once or twice a day.
Other users like to set up a specific time once each week with their therapist so that they can message each other in real time (sort of like a face-to-face session would be).
The best thing to do is talk to your therapist about your schedule and your needs, and see what will work best for both of you.