This will depend on how much you want to achieve!
Aside from your weekly 20 minute checkin calls, the amount of time will be up to you.
Some weeks may require more time than others, depending on the tasks ahead. Some weeks will be busier that others.
We all know that life sometimes gets in the way as well, so it's not really possible to specify this.
One of the things your coach can help with is finding more time. If that's a priority then make sure you discuss this with your coach at the start.