Right now its 40 which feels a little too long, as the games can get a little boring towards the end.
Also, the kill per game potential is significantly higher than in other modes, which I feel takes away from the game in general by making kill PBs feel pretty odd. If I set a PB in Invasion I will feel less of a drive to try to beat it in other modes because I know it will be so much harder to. It being so much easier to get high kill games in Invasion leads me to want to separate my invasion pbs from my non-invasion pbs, which makes both a lot less satisfying.
There already being a 300+ kill invasion game one day after its release really speaks to this issue. Based on the kill counts I have seen so far I think putting it at 30 minutes would make it more in line with existing modes in terms of kill potential.