Messenger allows unlimited, free-flowing conversations between members, all for 1 credit per conversation.
This means you can message back and forth with one member for as long as you want and only use up one credit, and conversations do not expire.
The number of messages you can send depends on your Premium membership level.
Silver members send up to 4 new messages daily
Gold members send up to 12 new messages daily
Platinum members send up to 25 new messages daily