Video calls on Instantgo get charged by duration of conversation or in any way you would prefer. When someone contacts you and want to do a video call, indicate the amount that you would like to receive to provide the service over video call. Once accepted, simply request the payment by clicking on the $ at the top right of the chat window. Once you have confirmed payment (in that same window), the option for video call will be enabled for the user to reach out to you.
When you are done with the service, simply click on Submit rating (in the $ window), to close the conversation and disable the video calls. You can resume them whenever you want. You control entirely that process.