Make paying mandatory after 45 days of use for new users, keep it free for the current users. Ask for a monthly fee. The only way to know if users are willing

Make paying mandatory after 45 days of use for new users, keep it free for the current users. Ask for a monthly fee. The only way to know if users are willing to pay for a service is to make it mandatory to pay and see if people pay, as long as you refuse to do that you will never know :)