not long enough to reply
I would guess about 60 seconds, all I know is that before I clicked to check the "remember me" box at the login prompt nothing I wrote got posted, even a two sentence short reply. The replies on threads don't get posted, but it seemed like private messages would go through and show up in the person's inbox even though the same time out error would pop up and tell me I must be logged in to do that.
I didn't realize the private message I had sent actually did get sent until the next day when I they wrote me back.
I wouldn't try to post anything without copy and saving it first if the remember box is not checked.