How should I time and structure an IB Paper 1?

You should ideally time your responses according to the amount of marks a question is worth. The final question is worth the most marks (9) so you should save at least 20 minutes to plan and write an answer to this, it would be even better if you saved 30 minutes. You can then dedicate ten minutes to answering each of the first three questions.
Structure wise, keep it simple! The amount of marks for parts A and B of question one tells you how many points you are required to make, but aim for one more than the requirement to be sure. For the second question, a good quiding structure is to have two paragraphs consisting of three sentences each. One paragraph should cover limitations and the other values. Each sentence should reference either Origin, Purpose, or Content with one sentence for each per paragraph, alternatively you can have three mini-paragraphs covering one of these and addressing a value and limitation for each. The compare and contrast is tricky to structure in a way that provides a running comparison– something vital to achieve top marks. I found that two paragraphs of three sentences with each covering either three similarities or three differences ensures a good running commentary. The final question is easiest to complete using a standard 5 paragraph essay including your introduction and conclusion using a table that lists three points as body paragraphs under which sources you can reference as evidence, and the snippets of your own knowledge are listed. Plan your introduction and conclusion around this information and then start writing. A short, detailed plan constructed in 5 minutes will ensure you are concise and do not waste any of your limited time.