module Devise::Models::Confirmable

confirmation_period_expired? # will always return false
# confirm_within = nil

confirmation_period_expired? # returns true
# confirm_within = 3.days and confirmation_sent_at = 4.days.ago

confirmation_period_expired? # returns false
# confirm_within = 3.days and confirmation_sent_at = 2.days.ago

Examples:
Checks if the user confirmation happens before the token becomes invalid

def confirmation_period_expired?
  self.class.confirm_within && self.confirmation_sent_at && (Time.now.utc > self.confirmation_sent_at.utc + self.class.confirm_within)
end