Had a fairly young system as Sophons. Cravers came and won the ground battle.  I got a message that I could take the system back in a few turns.  I had a fleet 3 turns away and as soon as they got to the system, it was already converted to the cravers.  Is that supposed to happen that fast?  

Yep. Basically any system the Cravers can launch a full ground assault against is hosed ATM; hopefully they tweak that soon so it requires a few turns of sieging first. Best advice I can give is post exploration scouts at the chokepoints to your colonies and move your fleets to intercept the moment the Cravers breathe in your direction.