I had an Alliance with the Terran Resistance. After a while, I got a popup stating "an agreement with the Terran Resistance has ended". I checked and the Alliance was dissolved. I was able to quickly and easily re-establish the Alliance, but this is annoying.
An Alliance shouldn't automatically expire. I don't know of a single case in history (though, admittedly I don't know ALL of human history) where an Alliance automatically expires. I don't believe that is the case with NATO, for example.
At any rate, it is annoying to have to do this. If I want to break the Alliance or if my Ally wants to, then we should be able to do that, but to automatically expire just doesn't make sense to me.