How often should I ask new video questions?

This is entirely up to you! You can manage expectations in your invites and in your panel description. You could say something like; “We normally discuss a few questions per month”. The panelists will not expect or enjoy a constant flow of questions. You can also use the video format to let people know things like; “After this question, we will take a break over the summer, we will be more active in August again”.