…the appropriate time to search for representation is when you literally can no longer manage shooting and client requests and calendars and making estimates and negotiating various licenses and shoot deliverables all at once.
The other time an agent is helpful is if you’re extraordinarily talented but a recluse, and want someone to be your “face” and leave it up to you to just make photographs. But the key thing here is that you need to be extraordinarily talented. Extraordinarily. Talented.