HTTP Long Polling - Timeout best practice
Maybe its your grasp of English which is the problem, but its the lifetime of the connection (time between connection opening and closing) you need to worry about more than the timeout (length of time with no activity after which the connection will be terminated).
Despite the existence of websockets, there is still a lot of deployed hardware which will drop connections regardless of activity (and some which will look for inactivity) where it thinks the traffic is HTTP or HTTPS - sometimes as a design fault, sometimes as a home-grown mitigation to sloloris attacks. That you have 3G and 4G clients means you can probably expect problems with a 5 minute lifespan.
Unfortunately there's no magic solution to knowing what will work universally. The key thing is to know how widely distributed your users are. If they're all on your LAN and connecting directly to the server, then you should be able to use a relatively large value, however setting the duration to unlimited will reveal any memory leaks in your app - sometimes its better to do refresh every now and again anyway.
Taking the case where there is infrastructure other than hubs and switches between your server and the clients, you need to provide a mechanism for detecting and re-establishing a dropped connection regardless of the length of time. When you have worked out how to do this, then:
dropped connections are only a minor performance glitch and do not have a significant effect on the functionality
it's trivial to then add the capability to log dropped connections and thereby determine the optimal connection time to eliminate the small problem described in (1)