Sorry about that, I can imagine it to be more than a bit annoying.
I haven't had it happen to me yet, despite being a slow PM writer, maybe I was just lucky.
But yeah, it seems there's an 'unused session timeout' in the server settings, currently it's set at the SMF default of 2880 seconds (48 minutes), which more or less corresponds with your experience. I will have to look it up a bit more to see if the limit's necessary, or if it'd be enough to simply increase it.