The reality is that the Prime Minister can call a snap election whenever and for whatever, subject to approval by MPs under the Fixed-term Parliaments Act 2011. If it is thought that the ruling party can become more dominant or lance some internal boils by calling an election then this is the route they will follow. It does require supreme confidence in the outcome however. With dodgy dave's referendum fiasco I would say calling an election early could give rise to totally unexpected results. All we need now is for the Prime Minister to resign her seat andd we will know that brexit is doomed.