For the past year or so, I have found that if I leave a 'tail -f' command running in my terminals for more than a few minutes, the SSH session will disconnect. Before the disconnection occurs, the terminal starts to become sluggish in so much as pressing Ctrl-C to cancel the tail command takes progressively longer to respond, the longer I leave the tail running. After the Ctrl-C is eventually handled, all is fine again.
Without a tail command running, the sessions will remain active indefinitely. I have not made any config changes to sshd and do not have any TMOUT setting in my terminal.
Google and SO have not yet revealed anything helpful to me on this.
I am presently using Ubuntu 18.04, and am connecting via Putty.