Prime Minister’s questions take place every Wednesday when the House is meeting, at 12 noon. They last at least half an hour.
They usually take the form of ‘engagements’ questions, which ask the Prime Minister to list the official engagements for that day. In practice, this means that you can ask the Prime Minister a question without the text of that question being printed in the Order Paper.
It is possible to table a 'substantive' oral question to the Prime Minister instead. This means that the text of your question will be printed in the Order Paper.