I give ChatGPT hints (eg past writings, specific formatting) and I am curious how long it will keep this in session. I assume it lasts as long as that chat window is in history? Asking because I want to continue to use a chat session over time without having to re-prompt it. Hopefully, this makes sense. 
Thx!
Not exactly.
Many users expect that and then learn that there are limits to how much is passed to the next part of the conversation.
My current understanding is that at the start of a ChatGPT conversation it has nothing known from other conversations. At the end of the first prompt/completion there is some knowledge but that knowledge is only in the prompt/completion, AFAIK it is not stored permanently.
Then for all further prompt/completion ChatGPT will summarize the knowledge from the previous prompt/completion and pass that summary along to the next prompt. Think of it as memory that can hold only the last item placed in it, that being the summary of the last prompt/completion. Also the size of the memory for the summary of a prompt/completion is also limited, so if you have lots of information some of it will just not make it into the summary and be lost.
If you look on this site you will see many questions about understanding this passing of knowledge for ChatGPT. Since I am a programmer understanding this limitation and knowing how to make the most of it comes naturally but can be very frustrating for those that don’t understand this.
However if you use the API and can incorporate an independent store of knowledge from the conversations then you can achieve your goal. There are post here that explain this in more detail and even sites that do such but as I don’t use either can’t give any references.
You can extend the memory of AI like ChatGPT with a technology called “embeddings”. Basically you can store conversations in a database and when you talk to the AI it will find the relevant information to the conversation to provide it to the AI.
Just search through the forum “embeddings” for more information
I built an “infinite memory chatgpt” here Infinite Memory Chatgpt - a Hugging Face Space by differentai