We ran into this and it came down to the client's time being out of sync with the server. First ensure your server's time is correct, https://portal.smartertools.com/kb/a3539/random-or-semi-regular-webmail-logouts.aspx?KBSearchID=886466. Then have clients verify their time using an online source like this https://time.gov/. Then depending on your user's environment they'll have to update the time on their DCs or individual PCs.

Also ask whether the client has a web filter that could be interfering. I believe SM uses both https and web sockets.

In addition to the excellent suggestions above, I recommend using this KB article here as a reference point for troubleshooting as there are some other possibilities for root causes:

Just to follow-up... checked the date/time.. the user's date/time and time zone that i observed.. is not an issue (they were off by 4 minutes).
The problem that I am having is exactly as 
https://portal.smartertools.com/community/a93906/users-being-logged-out-of-webmail-after-a-very-short-time.aspx

Merle - a time discrepancy of 4 minutes will definitely cause session handling issues in the browser. Has this user experienced the issue since you corrected the time? The other thing to try is configuring their time manually instead of using an automatic (NTP) update server. Has this been tried?

Have you tried to verify that the time is correct on the server itself, where SmarterMail is running. 

- not the time zone 

- just that the clock is not off and effecting the refresh/auth tokens

This has fixed many problems from webmail log outs to multi factor auth apps codes timing out

Thank you