You can set the number of free messages that other users can send to you.
How to Monetize Chat
My Page > Settings > Chat Settings > Message Settings > Select Option > Done
1)Allow only your fans to Chat
Only fans who have sent you a gift or purchased your paid media can send you a message.
2)Allow all Chats (Limited number of Free Messages)
All users can send you free messages for as many times as you set. ^ (Ex: 5)
3)Allow all Chats
Anyone can send you a message for free.