Let me check the settings. It is my understanding that you can take as long as you like to type in and modify a post, but once you have posted you have 30 minutes to read the post and open it again for editing.
The reason is that the "conversation" gets very strange if someone responds to what you have posted and then you edit what you said with the result that their response becomes nonsense.
There is also a session timeout for someone who walks away from their computer (or one in a hotel lobby) that is supposed to terminate after one hour of no activity. If you are a slow typist, you might take more than an hour to compose a post. If all you are doing is typing, the server does not know you are there and may log you off after an hour. If you do type very slowly, you can click on something from time to time to let the server know you are still around. One can also set the browser to "keep alive" but that is a very techie kind of thing to do and, in my opinion, is more hassle than it is worth.