The key for long articles is the memory allocated, which is expressed in terms of tokens. ChatGPT 4 by default has a total of 8K tokens that will result in around 800 words articles at most (if you give enought prompt that its easy for chatgpt to cover that 800 words).
If you need anything longer than that, there are multiple methods:
1. Applying for the 32K ChatGPT4 model
2. Using a different model like Claude2 (as already mentioned)
3. Using the ChatGPT API and creating a series of prompts, problem is that there will be a ton of repetitions, because the memory from the original prompt will be exhausted. This is by far the worst method, but the one that most sites do. Some people try to fine tune the multiple prompt approach in order to get significantly different results (for example, just by changing the tone of the prompt, you may generate drastically different answers, so if you keep asking a different tone per prompt you could end with a multiple 800 x number of prompts long-article.