Very simply as long as he can pass an amendment on the Queen's Speech, and no one can govern effectively.
At the start of every parliament, the Queen will deliver a speech outlining the new policies and laws of the government, MP's will debate this for around 5 days before voting on it. However this voting is largely symbolic, as if the government loses a vote on the Queen's speech, the Prime Minister usually resigns, as they are officially unable to form government.
As the official parliament website says
If an incumbent Prime Minister fails to form an administration or loses the vote on the Queen’s speech, the party or parties likely to be able to form an administration are asked to do so. Only once they have shown they can command the confidence of the House could they properly seek and be granted a general election.
What this basically means is that if Corbyn can pass an amendment to the Queen's speech, the government collapses and either he becomes Prime Minister and writes up his own Queen's speech forming his own government, or somebody else in the conservative party becomes the Prime Minister, writes up a new Queens speech and tries to form government again.
As such if neither Labour nor the Conservatives can govern effectively, then under the Fixed Term Parliaments Act, parliament can be dissolved and another general election can be called if either of the following conditions are met
- if a motion for an early general election is agreed either by at least two-thirds of the whole House or without division; or
- if a motion of no confidence is passed and no alternative government is confirmed by the Commons within 14 days.
Of course very strictly speaking if all the conservative MP's refuse to dissolve parliament, then no election will be held this year, however under the current conditions if the government can't pass much legislation they may wish to go back to the country to seek a proper mandate.
But even if all the conservative MP's refuse to dissolve parliament, if the rest of the house as a majority pass a bill calling for no-confidence in the government, and Corbyn is unable to form an alternative government within 14 days. Parliament is dissolved and a general election will be called.