She could refuse, but she can't. Hiw can an unelected Prime Minister refuse an elected one who has a manifesto pledge to adhere to?
She won't refuse, she will try to set the timing but that again is a dangerous game and will play into the hands of proof that regardless what we vote for means nothing if Westminster doesn't agree.
The ref should be after the deal is known, which will obviously be before we actually leave in two years. To think that we will hang about a further two years is not going to happen.