My data set has a column that indicates the time taken (in days) for members on a site - each with an ID - to sign up for an event. This can range between 1 to 300 days, with about half of the rows filled with NaNs for members who have yet to sign up for any events. Another column indicates the channel through which members initially joined the site, for example through direct referrals, a forum, or other websites.
The question is: If I'd like to try and estimate which groups I should target, as well as the best time, to send an e-mail reminder that increases the chances that members sign up for an event, what would be the appropriate approach if I have no labels for whether past e-mail reminders is successful or not? I was thinking of taking the average (median) of days taken to sign up for an event grouped by the channel through which members joined the site, but that would remove half the data which seems like a waste. Is there an alternative method?