Not sure what the minimum is as I do not want to spend too much time writing
My teacher always says write at least a page and a quarter, if you can write more and you have time though, you should! She corrects the exams every year and says that examiners are looking for 8 SRF's ( Significant Relevant Facts) BUT she advices to always put in 10 as you don't know which facts the examiner will accept. As long as you have the 8/10 the length shouldn't really matter as much!
it doesn't matter how long the essay is as long as it has 8/10 SRF's it doesn't matter.My teacher always says put down at least 10 SRF's.You don't get marked on how long the essay is only what related to the topic begin asked!