Acumatica actually made an improvement here since I made this original post.  We now have control by unchecking this box and setting it as we like.  Thank you Acumatica!

Additional information about system timeouts can be found here: 

Per our VAR, the timeout default for the cloud based solution is either 45 or 60 minutes. We requested that they increase ours to 90 minutes. (Done through a service request to Acumatica support as it requires a restart of our site. They were able to do the update and restart overnight.)

Note: Users with multiple windows open may still experience timeout messages if they are not actively using a given window. This generally doesn’t require them to login again - can fixed by refreshing the page.

I would note that this is an annoyance of the current client-server model in Acumatica (timeout if no I/O between client and server, even if user is actively engaging with the the site). This is another place where SignalR can come to the rescue with minimal coding overhead...