24 hours is WAY too long. Even an hour is too long. I rather like instant. If I am playing NOW I dont have time to wait 24 hours. This is what clans and alliances are for, to help defend your planets.
I also think clan diplomacy needs to be way better defined, but there just aren't enough players. A way to fix this is to make clans very small, no more then 5 people, which I think would be best, then add more importance to clan diplomacy.
But like I said, right now, just not possible. Not enough people play, or clans are just too big.