Yes, there’s a limit. Right now it’s 4000 tokens (gpt4 has a 32k model but i don’t think it’s available yet, at least with chatgpt). Once you near the limit, chatgpt will start dropping previous messages to stay under the limit. I don’t know what their algorithm is for deciding what to drop. It could be as simple as dropping the oldest stuff. Or maybe taking a long message and replacing it with a summary. But at a certain point, the conversation becomes “lossy”.