What produces a better response: emails sent in rapid fire vs a drip campaign?

We have a transactional email ("You've received a friend request") that is sent to users when they receive a friend request on our platform. It is important for us that user takes action on that email.

Assuming that most users do not respond because the email gets lost in their inbox, and baring in mind that we want this action to be done ASAP, what is a better strategy at reminding the user of the outstanding action?

• Send emails in a short interval, e.g. First email when the action is done, another one 24 hours if user does not respond, and another one 48 hours.
• Send emails in a long interval, e.g. First when action is done, second a week later, another one a month later.

pros/cons of the first approach:

• (pro) User is more likely to take action quickly.
• (con) User might unsubscribe from the service if we send too many emails.

pros/cons of the second approach:

• (con) User is unlikely to take action for a long time. Friend request might lose some relevance.