Before it can actually happen, she needs 2/3s of MPs to agree, according to the Fixed Term Parliament Act. Otherwise, it's a five-year term whether the current PM likes it or not.
This is an odd one, because if we have the election then it's likely to be a Tory landslide, but the Tories have only a narrow majority in the House of Commons and so need the turkeys to vote for Christmas.
On the other hand, Labour is currently led by Jeremy Corbyn, so it's entirely possible that enough of the turkeys will vote for every day of the year to be Christmas.