The board is programmed to automatically log you out after a certain period of time, but it shouldn't be the time it takes to write a long post. I will have to look up the exact time... but I think it MAY be on your side as a security precaution.
Can I ask what browser you are using?
It is a common recommended practice that if you are going to take a while to write a post due to it's length, then using a word processor or even notepad to write it is a good idea. Reason is, most boards will log you out if you are 'idle' for some time. Granted, you aren't idle... but the computer obviously doesn't see you typing, therefore it logs you out.
Unfortunately, it's more common for people to not log themselves out, therefore causing a security issue, vs. those taking a long time to write a post, so the precaution is needed (for your protection, as well)