With instant showtimes, you can always show your prospective customers a showtime that is close to the time when they select a showtime - no matter when they go to the landing page. This way, you avoid losing webinar participants by waiting too long for the next showtime without having to define a large number of regular showtimes. Whether instant showtimes are activated or not can be set in the menu "Showtimes" in the respective webinar project.
Define a maximum of 2-3 regular showtimes per week and additionally an instant showtime for the next full or half hour. Set the maximum number of displayed webinar showtimes to 3 or 4 accordingly. This way you ensure that there is always a short-term date but the dates are still rare enough to convey a high prestige.