Validating products that are not used daily

Context:

• You've done some research
• Spoke to users
• Built an MVP
• Promoted it
• A good number of users, say 40% even performed a critical action

And now to further prove your idea you would love to see retention... but your product is not for daily use. It might not be used even any other day. It may not be even used once a week. But more like once a month.

So how would you continue validating it? Waiting for a month to see retention seems to long when you're just validating your idea.

How do people go around validating the success of their MVPs which are not for a daily use?