My understanding was that the "Invasion" was a constant thing but that there would be a front. Em-8er isn't intended to emulate Firefall's Thumpdump invasions. Is the OP referring more to surgical strikes inside enemy territory? I would absolutely NOT want something like to be on a timer. It needs to be based on proximity to the front line and activity of the base itself.
let's assume that our version of Copa, that is the starting base that cannot be invaded, is on the north pole (arbitrarily). Then you have 5 player-controlled bases beyond that, let's say one of those is in a zone further south (duh). Now, the question I am posing is when the shifters attack that base, and we successfully fend off the invasion, how much time should pass before they try again, and if we fail after that amount of time they will try to take another base. The number of people online will probably affect both the difficulty of invasions and how many invasions there are at one time, but even if there are, say, 1000 people on, there should still be time between each attempt at taking our bases.
Make sense now?