Although I don't fully understand the settings, all of the VoIP accounts I have set up have suggested using 'Auto' as the option for keep alive mechanism and keep alive period.

That said, if you are worried about battery usage (as I am!), you can just create a VoIP profile in 'My Availablity' that logs you into your accounts when you switch to it.

This means that you cannot receive or make any VoIP calls when you are not online, but it can be helpful as I don't usually receive inbound calls to any of my VoIP accounts.