It's usually 90 minutes. Basically what happens is, at the 100,000-signature point, the leader of the House of Commons writes to the chair of the backbench business committee informing her that the petition has reached a threshold. The backbench business committee then has to decide if and when it will schedule a debate on that issue. Now its time is allocated to it by the government, and it's a set number of hours each year and it has to decide—
On November 25th, 2014. See this statement in context.