[fyfy18]

TCP connection can be open and idle for hours without keep alives. This can easily be configured with kernel parameters:

https://www.tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepaliv...

I suspect the hard bit for normal apps is keeping the app active on the device while it is in the background. Google has their FCM receiver running as a low level service so can make sure it is always running.

[ktjfi]

You can configure it locally in your kernel, but your ISP will most likely close your connections for you, especially if you're behind a CGNAT.

[londons_explore]

Even if you have native IPv6, some carriers do connection tracking anyway to prevent random port scans from the internet waking up your phone or trying various exploits.